There must be a causal link between the receipt of the benefit by the defendant and his or her change of position, so that, but for its receipt, the defendant’s position would not have changed. The leading case on the interpretation of causation for the purposes of the defence is Scottish Equitable plc v Derby. In that case the defendant had invested money in a pension policy with the claimant life assurance company. The defendant took early retirement benefit, but this was not recorded in the claimant’s records. The claimant sent the defendant a statement showing a fund of ?201,000 and subsequently confirmed this. The statement should in fact have read ?20,500. Consequently, the defendant received ?11,000 more each year than he should have done and he used this money to reduce his mortgage and the rest was spent on making modest improvements to his lifestyle. The defendant also faced straitened financial circumstances by virtue of the fact that he was in the process of separating from his wife. The claimant sought restitution and the defendant pleaded change of position. The Court of Appeal considered the appropriate test of causation and referred to the distinction between the wide and the narrow view of the defence as propounded by Burrows. According to the narrow view, the defence will only arise where the defendant has relied to his or her detriment on the validity of the receipt of the enrichment. According to the wide view, however, the defence will be available whenever the defendant’s position has changed so that it is inequitable to make restitution, but a causal link is still needed between the receipt of the enrichment and the change of position. The Court of Appeal preferred this wide interpretation of the defence. Consequently, it is not necessary to confine the defence to cases where the defendant relied on the validity of the receipt, although in many cases such detrimental reliance can be established. Rather, it is enough that the receipt of the enrichment caused the defendant to change his or her position. The Court further suggested that the appropriate test of causation was at least the ‘but for’ test.  In other words, but for the receipt of the enrichment the defendant’s position would not have changed. It followed that, on the facts of the case, financial difficulties arising from the defendant’s separation was not causally linked to the mistaken payment and so did not constitute a change of position.
According to this wider interpretation causation can be established in two ways: first, where it can be established that the defendant no longer retains the specific benefit which was received from the claimant, or, secondly, where the defendant’s position has changed in other ways as a result of his or her reliance on the validity of the receipt of the benefit from the claimant.1 0 The reason for distinguishing between the two tests of causation is simply because the second test depends on proof that the defendant detrimentally relied on the validity of the receipt to change his or her position, whereas the first test does not require any proof of reliance on the validity of the receipt.
In Test Claimants in the FII Group Litigation v HMRC (No 2) Henderson J recognized that the defence is essentially concerned with disenrichment. A relevant consideration is how easy it would be for the defendant to reverse his or her enrichment but, unlike the approach adopted in Australia and advocated by Bant, the test is not simply one of irreversibility, since the change of position must also satisfy the but for test of causation.
(1) Loss of Benefit Test of Causation
According to the lost benefit test, the defence is open to the defendant if he or she no longer retains the actual benefit which had been received from the claimant. So, for example, if the claimant paid ?1,000 to the defendant by mistake but this money was stolen from the defendant when walking home, the defence should in principle be available. Whether the defence will actually succeed would then depend on whether the circumstances are such that it would be inequitable to require the defendant to make restitution to the claimant, but the fact that the money was stolen will probably be sufficient for the defence to operate.132 Similarly, if the enrichment is destroyed, for example by fire, the defence should in principle be available.
(a) Establishing that the Benefit Was Lost
The essential feature of this test of causation is that the defendant no longer has the benefit which he or she received from the claimant. This satisfies the but for test of causation because, but for the receipt of the benefit, the defendant would not have lost it and, by losing it, he or she has suffered a detriment. Although the test of causation is formulated in terms of whether the defendant has retained the benefit which was received from the claimant, it does not follow that this aspect of the defence is only relevant to proprietary restitutionary claims. Even where the claim is founded on the reversal of the defendant’s unjust enrichment it may be defeated by the fact that the defendant no longer retains the enrichment which was received, simply because the fact that the defendant no longer has the enrichment may make it inequitable to require the defendant to make restitution.
(b) Retention of the Product of the Benefit
If the defendant has retained a product of the benefit, for example where he or she uses money received from the claimant to buy a car, it should not be possible to conclude that the defendant has lost the benefit, because he or she has retained the value of the original benefit, albeit in a different form. In RBC Dominion Securities Inc v Dawson, however, the Newfoundland Court of Appeal held that the defence of change of position could apply even where the defendant had substituted property for the original benefit received from the claimant. But this is surely inconsistent with the essential feature of the defence, namely that it is only inequitable for the defendant to make restitution to the claimant to the extent that the defendant’s position has changed. Where the defendant has merely substituted one benefit for that received from the claimant it is not clear why the defence should succeed, since the defendant has retained the value ofthe benefit so that he or she has not necessarily suffered any detriment. It is surely detriment suffered by the defendant which makes it inequitable for the defendant to make full or any restitution. So, for example, where, as occurred in Scottish Equitable plc v Derby, the defendant has used the money which he or she had received to discharge an existing debt which he or she would have had to pay anyway, the defence will usually not be available because the defendant has not suffered any detriment and remains enriched. Similarly, where money has been used to purchase an item of value, the defendant remains enriched to the extent of the value of that item.
(c) Fall in Value of the Benefit
If the defendant receives an asset from the claimant, or receives money which the defendant uses to purchase an asset, and the value of that asset falls, the defence should be applicable to the extent that the value has fallen.  That the defence of change of position may operate in this way is probably illustrated by the decision of the Court of Appeal in Cheese v Thomas,142 where the claimant and the defendant had jointly contributed to the purchase of a house. This transaction was set aside on the ground of undue influence, and the question before the court related to how the relief should be determined in the light of the fact that the value of the house had fallen. It was held that this loss in value should be borne by both parties in proportion to their respective contributions to the purchase of the house. Although the court justified this loss apportionment by reference to an inherent equitable discretion to do practical justice, Chen-Wishart has argued that the result is consistent with the application of change of position, because, although the defendant was unjustly enriched by the receipt of money from the claimant, this was ‘reduced by the subsequent loss to the value of the house in the recession’. Consequently, the result of Cheese v Thomas suggests that the change of position defence may be applicable even where the value of the enrichment received from the defendant has fallen, but, as always, this will only be the case if it is equitable for the defendant to rely on the defence.
(d) The Change of Position Must Be Extraordinary
Although it appears that the causation test is easily established where the defendant no longer has the benefit which he or she received from the claimant, this is not the case. There is an additional requirement which also needs to be considered before causation is established, namely that the loss of the benefit did not occur in the ordinary course of events; the loss must be extraordinary. The reason for this requirement is illustrated by the following example. If the defendant received a sum of money by mistake from the claimant and the defendant used this money to defray ordinary living expenses, it is not possible to conclude that, but for the receipt of the money, the expense would not have been incurred, since if the money had not been received the defendant would have paid the expenses from his or her existing resources.  Consequently, in such circumstances the test of causation can only be satisfied where the expenditure is extraordinary. This requirement was specifically recognized by Lord Goff in Lipkin Gorman.146 It follows that the defence will only be available in exceptional cases. 
In Test Claimants in the FII Group Litigation v HMRC (No 2)148 Henderson J recognized that an extraordinary change of position does not require proof that the change of position was unusual, either intrinsically or for the particular defendant. It is sufficient that the change of position would not have been incurred but for the receipt of the enrichment. In other words, the language of extraordinariness simply embodies a test of but for causation. Consequently, increased expenditure of a routine nature can count, as long as it can be shown that the expenditure would not have been incurred but for the receipt of the enrichment.
The defendant’s change of position will clearly be considered to be extraordinary where he or she no longer has the benefit for reasons outside his or her control. So, if the benefit has been stolen or destroyed, the change of position should always be considered to be extraordinary. The test of extraordinary change of position will be most relevant where the benefit is money and the defendant has spent it. In such a situation the defendant’s expenditure will be extraordinary where, for example, he or she engaged in a special project or undertook a special financial commitment as a result of the receipt of the benefit from the claimant.
The test of extraordinary change of position is assessed by reference to changes in the defendant’s particular circumstances. It follows that if the defendant uses money received from the claimant on normal living expenses this can still be considered to be extraordinary expenditure if the defendant would not have incurred the expense but for the receipt of the money from the claimant. A similar test is adopted in respect of the defence of estoppel, as was seen in Avon County Council v Howlett, where the defence was established even though the defendant had spent money on everyday items, because he would not have purchased them if he had not received the money. That a similar approach can be adopted in respect of the defence of change of position is illustrated by the decision of the Newfoundland Court of Appeal in RBC Dominion Securities Inc v Dawson, in which the defence applied to the extent that the defendant had used the money received from the claimant on clothes and furnishings which she had bought or refurbished only because she would not have done so if the money had not been received.
(e) Acquisition of a Cause of Action Against a Third Party
Where the consequence of the defendant’s change of position is that he or she acquires a cause of action against a third party, can the defendant really be considered to have suffered detriment? For example, if the claimant pays ?1,000 to the defendant by mistake and this money is stolen by a thief, it might be concluded that the defendant’s position has not changed because he or she has a restitutionary claim against the thief to recover the money. If the defendant has actually recovered the money from the thief it is obvious that the defendant’s position has not detrimentally changed. But this is the easy case. What if the claimant sued the defendant for restitution before the defendant had sued the third party? Should the fact that the defendant has a potential claim against the third party mean that his or her position has not changed? Since the success of the defendant’s claim is speculative, the preferable view is that the defendant should still be considered to have suffered a detrimental change of position, so that he or she will not be required to make restitution to the claimant. The claimant may, however, seek the remedy of subrogation whereby the defendant’s cause of action against the third party can be vested in the claimant.154 If this remedy were not awarded the defendant could be unjustly enriched at the claimant’s expense if the defendant did eventually recover the money from the third party. The consequence of this analysis is that the claimant ultimately bears the risk of the defendant losing the enrichment and being unable to recover it from the third party.
(2) The Detrimental Reliance Test of Causation (a) The Basic Test
The alternative test of causation is satisfied where, even though the defendant has retained the particular benefit which he or she received from the claimant, the defendant relied on the validity of the receipt of the benefit and suffered detriment in other ways. For example, if the defendant received ?1,000 by mistake and paid this money into a bank account, and, as a result of this receipt, sold some shares and used the proceeds to pay for a holiday, the defence should be available since, but for the receipt, the defendant’s position would not have changed. Although the defendant will presumably have enjoyed the holiday, this can be considered to be a detrimental change of position because he or she has lost value by relying on the validity of the receipt and so can be considered to be no longer enriched to the extent of that value.
This test of causation has been recognized in some jurisdictions,  and it was effectively adopted in Lipkin Gorman,156 where a thief had stolen money from the claimant and gambled with it at the defendant’s casino. The defendant was considered to have changed its position by paying winnings to the thief. The detrimental reliance test of causation had been satisfied because, but for the defendant’s belief that the thief was entitled to gamble with the money, it would not have paid the winnings to the thief.
In Scottish Equitable plc v Derby157 Robert Walker LJ identified a number of situations where the necessary detriment could be identified. For example, where the defendant voluntarily gives up employment as a result of receiving a mistaken payment, at an age when it would not be easy to get new employment. Or where the defendant enters into a long-term financial commitment, such as a ten-year lease, which is not easy to dispose.
Usually the defendant’s change of position will take the form of the defendant incurring expenditure as a result of the receipt of a benefit from the claimant. But the defendant’s position may change in other ways. For example, the defendant may have received money from the claimant in the mistaken belief that the claimant owed him or her the money, whereas it was actually owed to the defendant by a third party. If the defendant relied on the validity of the receipt from the claimant and did not discover that the money was owed by the third party until the statutory limitation period had passed, the defendant’s position will have changed, since he or she cannot recover the money from the third party. Consequently, if the claimant seeks restitution of the money from the defendant he or she should be able to rely on the defence of change of position. In Australian Financial Services and Leasing Pty Ltd v Hills Industries Ltd159 the High Court of Australia recognized that the defence was available where the defendant had relied on the receipt and suffered an irreversible detriment, which was established in that case where the defendant had decided not to pursue claims against the claimant following the receipt of a mistaken payment.
(b) General Hardship is Not Sufficient
It is not sufficient to satisfy this test of causation that the defendant suffered general hardship after receiving the benefit from the claimant, where the hardship does not relate to the receipt of that benefit. For example, if the defendant received ?1,000 from the claimant by mistake and a month later the defendant was made redundant or was the victim of a burglary, it will not be possible to establish change of position because the change in the defendant’s circumstances is unrelated to the receipt of the enrichment from the claimant. There is no causal connection between the receipt of the benefit and the detriment suffered by the claimant.
(c) The Relevance of an Extraordinary Change of Position
Although Lord Goff suggested in Lipkin Gorman that the defence could be established only where the defendant’s change of position was extraordinary, this should be considered to be a requirement only where the defendant relies on the first test of causation. Where he or she relies on the detrimental reliance test of causation it should not be necessary to show that the defendant’s change of position was extraordinary, simply because the defendant may be able to prove that he or she detrimentally relied on the validity of the receipt of the benefit even though the change was not extraordinary. Nevertheless, the fact that the change of position was extraordinary will make it much easier for the defendant to show that but for the receipt of the benefit the defendant’s position would not have changed. It follows that the extraordinary nature of the defendant’s change of position is a factor which suggests detrimental reliance, but should not be treated as a requirement in its own right.
(d) Non-Pecuniary Change of Position
In Commerzbank AG v Gareth Price-Jones the Court of Appeal recognized that a nonpecuniary change of position might be sufficient to establish the defence. In that case the defendant’s change of position in staying in his job, rather than seeking more lucrative employment elsewhere, was not considered to be sufficient for it to be inequitable to require him to make restitution of ?250,000 which his employer had paid by mistake. This was because there was no relevant connection between the anticipated receipt of money and the decision to stay in his job and that decision did not have a ‘significant, precise or substantial adverse impact’ on the defendant.164 But the Court would have been willing to allow the defence to succeed if this connection could have been established on the facts. Although this conclusion might appear to undermine one ofthe essential characteristics of the defence, namely that it operates only to the extent that the defendant can be considered to have lost the value of the enrichment, this is not the case. A non-pecuniary change of position can only be taken into account if it can be valued in some way and so a non-pecuniary change of position can be converted into a pecuniary one. So, for example, if a defendant has turned down a firm offer of a better paid job elsewhere as a direct result of receiving a payment from his or her present employer, albeit a payment received by mistake, this is a detriment which can be valued and, to that extent, the defendant will have changed his or her position. In fact, this analysis is inevitable since a change of position can only be taken into account by identifying the value of a detriment and then using that value to reduce or negate the value of the enrichment received. It follows that if the ‘non-pecuniary’ change of position cannot be valued, it cannot be taken into account as a sufficient detriment. So, if, for example, the defendant decides to start a family  as the result of receiving a mistaken payment of ?100,000, the value of this ‘detriment’ is surely so speculative that it cannot be considered to be relevant.
(e) Anticipatory Expenditure
In South Tyneside Metropolitan BC v Svenska International plc166 Clarke J held that, at least on the facts of that case, the defence was not available where the defendant had changed its position before receiving an enrichment. In Svenska the defendant bank had entered into an ultra vires swap transaction with the claimant local authority. The bank had also entered into hedging transactions to reduce substantially its potential liability to the local authority under the swap transactions. Since the swaps transactions were void the claimant sought restitution of its money and the defendant pleaded change of position on the ground that, in reliance on the validity of the swaps transactions, it had incurred losses by entering into the hedging transactions. Clarke J held  that the defence was not available because the swaps transaction was void. This was significant because, had the defendant received the enrichment before changing its position, it would be possible to conclude that the defendant had relied on the validity of the receipt. But where the change of position occurred before the enrichment had been received, the defendant could not rely on the validity of the receipt and could only rely on the validity of the transaction and, since the transaction was null and void, the defendant’s reliance was a nullity.
It remained a matter of some controversy whether the decision in Svenska was confined to the peculiar facts of the case or was of general application. The Privy Council in Dextra Bank and Trust Co v Bank of Jamaica168 subsequently concluded that, although the decision may be doubted, it is at the very least confined to its special facts, so that the defence of change of position may be applicable in respect of anticipated reliance. The Privy Council noted that the main justification for distinguishing between change of position before and after the receipt of a benefit is that ‘whereas change of position on the faith of an actual receipt should be protected because of the importance of upholding the security of receipts, the same is not true of a change of position in reliance on an expected payment, which does not merit protection beyond that conferred by the law of contract (including promissory estoppel)’. Their Lordships did not, however, find such reasoning to be convincing. They said:
it is difficult to see what relevant distinction can be drawn between (1) a case in which the defendant expends on some extraordinary expenditure all or part of a sum of money which he has received from the plaintiff, and (2) one in which the defendant incurs such expenditure in the expectation that he will receive the sum of money from the plaintiff, which he does in fact receive. Since ex hypothesi the defendant will in fact have received the expected payment, there is no question of the defendant using the defence of change of position to enforce, directly or indirectly, a claim to that money. It is surely no abuse of language to say, in the second case as in the first, that the defendant has incurred the expenditure in reliance on the plaintiff’s payment or, as is sometimes said, on the faith of the payment.
This approach has since been endorsed by the Court of Appeal in Commerzbank AG v Gareth Price-Jones.170 The approach of the Privy Council is essentially consistent with that of the Court of Appeal in Scottish Equitable plc v Derby.171 If the emphasis should now be placed on causation, it should make no difference whether the defendant’s position changed either before or after the enrichment was received. If the position changed before the receipt, the defendant will need to establish that he or she relied on the possibility of a future receipt to change his or her position. True, such a defendant is taking a risk that he or she might not receive the enrichment, but, as the Privy Council acknowledged, the defence will only be available anyway where the enrichment has been received, for otherwise there will be no basis for the claimant to bring a restitutionary claim.   
(f) The Standard of Proof
In Philip Collins Ltd v Davis173 Jonathan Parker J recognized that the evidential burden was placed on the defendant to establish the change of position, but he also recognized that the court should not apply too strict a standard of proof. In particular, it might not be appropriate to expect the defendant to produce conclusive evidence ofhis or her change of position, since the defendant would not have had any expectation of the need to prove this in court. In that case the claimant company had mistakenly overpaid royalties to the defendants, who were professional musicians. The question arose as to whether the defendants could rely on the defence of change of position. They were unable to show that any particular item of their expenditure was referable directly to the receipt of the royalties, but the judge did not consider this to be fatal on the facts of the case, because the approach of the defendants was to gear their outgoings to the income they received, and, because the royalties were received periodically, it was possible to conclude that the overpayments had caused a general change of position.
This was confirmed by the Court of Appeal in Scottish Equitable plc v Derby,174 which emphasized that it was not appropriate to impose too demanding a standard of proof when an honest defendant stated that he or she had spent an overpayment on improving his or her lifestyle, but was unable to produce detailed accounts to establish how this lifestyle was improved. The Court emphasized that the defence was not limited to specific identifiable items of expenditure. In the context of claims for restitution against public authorities, it has been recognized that, in respect of government expenditure, it is not necessary to show a precise link between particular receipt and particular items of expenditure.
-   3 All ER 818.
-  Now see Burrows, The Law of Restitution (3rd edn), 528-31.
-  This is the preferred view in Australia: David Securities Commonwealth Bank of Australia (1992) 175CLR 353, 385; State Bank of New South Wales Ltd v Swiss Bank Corp (1995) 39 NSWLR 350, 356-7; Citigroup vNational  NSWCA 381, 82 NSWLR 391, [5-6] (Bathurst CJ, Allsop P, and Meagher JA) and [64-65](Barrett JA); Australian Financial Services and Leasing Pty Ltd v Hills Industries Ltd  HCA 14, (French CJ), ,  (Hayne, Crennan, Kiefel, Bell, and Keane JJ).
-  See also Philip Collins Ltd v Davis  3 All ER 808, 827 (Jonathan Parker J).
-  Scottish Equitable plc v Derby, 827 (Robert Walker LJ).
-  See Bant, The Change ofPosition Defence, 144.
-   EWHC 4302 (Ch), . 132 See p 690, below.
-  RosevAIB Group (UK) plc  EWHC (Ch) 1737,  1 WLR 2791, 2806 (Nicholas Warren QC);MacDonald v Coys of Kensington (Sales) Ltd  EWCA Civ 47,  1 WLR 2775.
-  See Campden Hill Ltd v Chakrani  EWHC 911 (Ch),  (Hart J). The second test of causation,
-  namely detrimental reliance on the validity of the receipt of the benefit, may be applicable instead. See p 686,below. 135 (1994) 111 DLR (4th) 230.
-  Bant, The Change ofPosition Defence, 134, considers this to be an example of an irreversible change ofposition because the defendant could not easily find equivalent secondhand furniture.
-  See p 690, below. 138  3 All ER 818.
-  139 See also National Bank of Egypt International Ltd v Oman Housing Bank SAOC  EWHC 1760
-  (Comm),  1 All ER (Comm) 246; Credit Suisse (Monaco) SA v Attar  EWHC 374 (Comm).Although Robert Walker LJ did acknowledge in Scottish Equitable plc v Derby that paying off a debt might bedetrimental if the loan which was discharged was on advantageous terms:  3 All ER 818, .
-  Barros Mattos Jnr v MacDaniels Ltd  EWHC 1188 (Ch),  1 WLR 247, . See also CreditSuisse (Monaco) SA v Attar  EWHC 374 (Comm), .
-  See Lipkin Gorman (a firm) v Karpnale Ltd  2 AC 548, 560 (Lord Templeman).
-   1 WLR 129. See p 267, above.
-  M Chen-Wishart, ‘Loss Sharing, Undue Influence and Manifest Disadvantage’ (1994) 110 LQR 173.
-  Ibid, 178.
-  Barros Mattos Jnr v MacDaniels Ltd  EWHC 1188 (Ch),  1 WLR 247,  (Laddie J).
-   2 AC 548, 580. See also Westminster Bank plc v Somer  1 All ER 198, 213 (Potter LJ);Barons Finance Ltd v Kensington Mortgage Co Ltd  EWCA Civ 1592,  (Tomlinson LJ).
-  Lipkin Gorman (a firm) v Karpnale Ltd  2 AC 548, 580.
-   EWHC 4302 (Ch), .
-  Rural Municipality of Storthoaks v Mobil Oil Canada Ltd (1975) 55 DLR (3d) 1, 13 (Martland J).
-   1 WLR 605. See p 670, above. 151 (1994) 111 DLR (4th) 230.
-  152 Nolan, ‘Change of Position’, 170-2.
-  153 At least where there are significant practical or legal hurdles in pursuing the claim: Bant, The Change of
-  Position Defence, 142. 154 Nolan, ‘Change of Position’, 172. See p 20, above.
-  See, for example, the New Zealand Judicature Act 1908, s 94B as amended. Proof of reliance on thevalidity of the receipt is also required in Canada (Rural Municipality of Storthoaks v Mobil Oil Canada Ltd(1975) 55 DLR (3d) 1, 13 (Martland J)), and Australia (David Security Pty Ltd v Commonwealth Bank ofAustralia (1992) 175 CLR 353, 385 and Australian Financial Services and Leasing Pty Ltd v Hills Industries Ltd HCA 14).
-   2 AC 548. 4  3 All ER 818, 827.
-  defence of change of position was rejected in such circumstances. If these facts arose today the defence should
-  Where the ?1,000 itself is stolen in the burglary the defendant will satisfy the first test of causation sincehe or she no longer has the benefit. See p 683, above.
-   2 AC 548, 580. See also David Securities v Commonwealth Bank of Australia (1992) 175 CLR
-  3 5 3, 38 6. 3 Nolan, ‘Change of Position’, 62.
-   EWCA Civ 1663,  1 P & CRD 15. 164 Ibid,  (Mummery LJ).
-  To use an example suggested by Birks, Unjust Enrichment (2nd edn), 260.
-   1 All ER 545. 3 Ibid, 565.
-  168  UKPC 50,  1 All ER (Comm) 193. 5 Ibid, .
-  170  EWCA Civ 1663,  1 P & CRD 15,  (Mummery LJ), and  (Munby J). See also
-  Charles Terence Estates Ltd v Cornwall County Council  EWHC 2542 (QB),  (Cranston J).
-   3 All ER 818.
-  Bant, The Change of Position Defence, 156, defends anticipatory change of position by virtue of practiceand policy.
-   3 All ER 808. 174  3 All ER 818.
-  175 Ibid, 827 (Robert Walker LJ). 176 Ibid. 177 See p 410, above.
-  178 Bloomsbury International Ltd v Sea Fish Industry Authority  EWHC 1721 (QB),  1 CMLR
-  12,  (Hamblen J); Test Claimants in the FII Group Litigation v HMRC (No 2)  EWHC 4302 (Ch),
-   (Henderson J).
-  179 Lipkin Gorman (a firm) v Karpnale Ltd  2 AC 548, 578.