I don't have my books in front of me but the answer is that usual principles would seem to apply, see the recent case of Soulsbury v Soulsbury [2008] 2 W.L.R. 834-in very breif outline, the husband promised his wife a sum of 100K in his will if she did not enforce an order for periodical payments by him. He remarried, and after his death his wife refused to pay over the 100K to the ex-wife. Ex-wife sued (the estate) for breach of contract, and was successful on general contract principles.

 

"The judge's findings establish that there was a binding agreement between the claimant and the deceased entered into for good consideration. *839 There was certainty about the subject matter of the gift, namely, to arrange his affairs in such a way that on his death £100,000 would be paid to her. There is no appeal against that finding. Thus there appears no reason why the personal representatives should not give effect to his promise. Ordinarily where the promisee has honoured her bargain, the deceased would remain bound to honour his. If he failed to provide payment for her on his death, her right to that payment became enforceable by a direct right of action for breach of contract against his estate" [para 15]

 

And....

 

 

49 This is a classic unilateral contract of the Carlill v Carbolic Smoke Ball Co [1893] 1 QB 256 or the “walk to York” kind. Once the promisee acts on the promise by inhaling the smoke ball, by starting the walk to York or (as here) by not suing for the maintenance to which she was entitled, the promisor cannot revoke or withdraw his offer. But there is no obligation on the promisee to continue to inhale, to walk the whole way to York or to refrain from suing. It is just that if she inhales no more, gives up the walk to York or does sue for her maintenance, she is not entitled to claim the promised sum.

50 The facts of this case are analogous to Errington v Errington and Woods [1952] 1 KB 290 in which a father paid a lump sum for a house for his son and daughter-in-law leaving a balance payable by mortgage to a building society. He promised his son and daughter-in-law that if they continued in occupation and paid the mortgage instalments, he would transfer the property to them after the last instalment had been paid. When the father died, his personal representatives sought to revoke this promise and claimed possession. It was held that the couple were entitled to occupy the house as long as they paid the mortgage instalments. Denning LJ said, at p 295:

“The father's promise was a unilateral contract-a promise of the house in return for their act of paying the instalments. It could not be revoked by him once the couple entered on performance of the act, but it would cease to bind him if they left it incomplete and unperformed, which they have not done. If that was the position during the father's lifetime, so it must be after his death.”

The present case is stronger than Errington's case since on Mr Soulsbury's death, Mrs Soulsbury had completed all possible performance of the act required for enforcement of Mr Soulsbury's promise. "