Unilateral Mistake in UK Law

(i) there must be a common assumption as to the existence of a state of affairs; (ii) there must be no warranty by either party that that state of affairs exists; (iii) the non-existence of the state of affairs must not be attributable to the fault of either party; (iv) the non-existence of the state of affairs must render performance of the contract impossible; (v) the state of affairs may be the existence, or a vital attribute, of the consideration to be provided or circumstances which must subsist if performance of the contractual adventure is to be possible.

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.

But as the alleged common intention ex hypothesis contradicts the written instrument, convincing proof is required in order to counteract the cogent evidence of the parties' intention displayed by the instrument itself.

I would provisionally conclude that the true requirement is simply for there to be a causative mistake of sufficient gravity; and, as additional guidance to judges in finding and evaluating the facts of any particular case, that the test will normally be satisfied only when there is a mistake either as to the legal character or nature of a transaction, or as to some matter of fact or law which is basic to the transaction.

What is clear from these cases is that there is not, so to speak, a limit to the amount of red ink or verbal rearrangement or correction which the court is allowed. All that is required is that it should be clear that something has gone wrong with the language and that it should be clear what a reasonable person would have understood the parties to have meant. In my opinion, both of these requirements are satisfied.

Whether there was in any particular case knowledge of the intention and mistake of the other party must be a question of fact to be decided upon the evidence. Basically it appears to us that it must be such as to involve the defendant in a degree of sharp practice. Basically it appears to us that it must be such as to involve the defendant in a degree of sharp practice.

But I think it should be recognised that one is here concerned with a restitutionary remedy and that the appropriate questions are therefore, first, whether the defendant would be enriched at the plaintiff's expense; secondly, whether such enrichment would be unjust and thirdly, whether there are nevertheless reasons of policy for denying a remedy. But I think it should be recognised that one is here concerned with a restitutionary remedy and that the appropriate questions are therefore, first, whether the defendant would be enriched at the plaintiff's expense; secondly, whether such enrichment would be unjust and thirdly, whether there are nevertheless reasons of policy for denying a remedy.