How has the doctrine of mistake been interpreted in the law of equity?

In Associated Japanese Bank, Steyn J. also considered the development of the doctrine of mistake in the law of equity. He noted that law “has not stood still”, that a more flexible doctrine has been developed in cases such as Solle v. Butcher, [1949] 2 All E.R. 1107, [1950] 1 K.B. 671 such that “today, it is clear that mistake in equity is not circumscribed by common law definitions”. (At pp. 911-912). In Solle v. Butcher, Lord Denning, M.R. said: A contract is also liable in equity to be set aside if the parties were under a common misapprehension either as to facts or as to their relative and respective rights, provided that the misapprehension was fundamental and that the party seeking to set it aside was not himself at fault. [At p. 1120 in [1949] 2 All E.R.].