The defence of 'non est factum' is available to the contracting party who has committed mistake as to the :

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The defence of non est factum is most obviously important in two situations. The first is where a party has signed the supposed contract as the result of the fraud of a third party and the other party to the contract has no actual knowledge or reason to know of, the fraud. For example, in United Dominions Trust Ltd v Western’ the defendant signed a blank hire-purchase proposal form and the dealer filled in incorrect figures before dispatching it to the finance company. The second is where the fraud has been committed by the other party to the alleged contract or deed and a third party has then relied on the document. In Saunders v Anglia Building Society an elderly lady signed what she believed to a be a deed of gift to her house to her nephew but which was in fact an assignment on sale to a third party who mortgaged the house to the defendants and kept the proceeds. If the case is one of fraud or misrepresentation by the other party to the contract, with no third party involved, the majority in the Court of Appeal in Lloyds Bank Plc v Waterhouse’ said that the case should be dealt with as one of misrepresentation. Alternatively, where the other party knew that the document did not represent the intention of the party signing it, the latter may have a remedy for unilateral mistake.