Mistake of Fact in UK Law

First, there must have been a mistake as to an existing fact, including a mistake as to the availability of evidence on a particular matter. Secondly, the fact or evidence must have been "established", in the sense that it was uncontentious and objectively verifiable. Fourthly, the mistake must have played a material (not necessarily decisive) part in the Tribunal's reasoning.

At present I do not wish to state the principle any less broadly than this: that the defence is available to a person whose position has so changed that it would be inequitable in all the circumstances to require him to make restitution, or alternatively to make restitution in full.

On an appeal the court must decide any question of law arising from an award on the basis of a full and unqualified acceptance of the findings of fact of the arbitrators. It is irrelevant whether the court considers those findings of fact to be right or wrong. It also does not matter how obvious a mistake by the arbitrators on issues of fact might be, or what the scale of the financial consequences of the mistake of fact might be.

Corresponding to mistake as to the existence of the subject matter is mistake as to title in cases where unknown to the parties the buyer is already the owner of that which the seller purports to sell to him. To such a case Lord Westbury applied the principle that if parties contract under a mutual mistake and misapprehension as to their relative and respective rights the result is that the agreement is liable to be set aside as having proceeded upon a common mistake.

It is clear that any civilised system of law is bound to provide remedies for cases of what has been called unjust enrichment or unjust benefit, that is to prevent a man from retaining the money of or some benefit derived from another which it is against conscience that he should keep.

Cases where the payer was aware that there was an issue of law which was relevant but, being in doubt as to what the law was, paid without waiting to resolve that doubt may be left on one side. A person who pays when in doubt takes the risk that he may be wrong–and that is so whether the issue is one of fact or one of law. As for mistake, this may arise where there is no suggestion that the law has changed since the payment was made.

I would therefore hold that money paid by a citizen to a public authority in the form of taxes or other levies paid pursuant to an ultra vires demand by the authority is prima facie recoverable by the citizen as of right.