The final reason why the receipt of a payment might be considered to be ultra vires the public authority is because of a mistake which meant that, even though the public authority was authorized to demand payment of money in principle, the claimant was not liable to pay the money, either at all or at least not the full amount which was received. The mistake might be factual as regards the assessment of the sum which was due or a mistake of law arising from the misinterpretation of a particular regulation which was valid but was never intended to apply to such a payer.
In all of these situations, even though the initial payment to the public authority may have been ultra vires, it is possible for Parliament to pass retrospective legislation to validate the payment. Such legislation does not contravene the European Convention on Human Rights, but, at least if the receipt is ultra vires because of infringement of EU law, the legislation must comply with the principles of effectiveness and equivalence.
The key question which needs to be considered in this chapter is whether the person who made an ultra vires payment to the public authority can sue the authority for restitution and, if so, what the relevant ground of restitution is. Whilst this analysis will concentrate on restitutionary claims at common law, it will also be necessary to examine certain specific statutory provisions which recognize significant rights to obtain restitution of ultra vires payments from public authorities, which will prevail over a claim at common law.
-  National Provincial Building Society v UK  STC 1466. 30 See p 390, above. 31  UKHL 49,  1 AC 558. 32  2 AC 349. See Chapter9. 33 See Woolwich Equitable Building Society v IRC  AC 70 where the claimant had not been made amistake, either of fact or of law. See p 398, below. 34 See Deutsche Morgan Grenfell v IRC  UKHL 49,  1 AC 558. 35 See further p 737, below. 36 See p 409, below. 37 See p 409, below.
-  38 See Chapter 10. See generally N Enonchong, ‘Restitution from Public Authorities: Any Room for Duress?’in S Elliott, B Hacker, and C Mitchell (eds), Restitution of Overpaid Tax (Oxford: Hart Publishing, 2013), ch 4.