Wilmott v Barber (1880) - five probanda
1. Plaintiff must have made a mistake as to his legal rights
2. P must have expended some money or have done some act on the faith of his mistaken belief
3. D, the possessor of legal right, must know of the existence of his own right which is inconsistant with the right claimed by the plaintiff
4. D must know of the plaintiffs mistaken belief of his rights
5. D must have encouraged P in his expenditure of money or in the other acts which he has done either directly or by abstaining from asserting his legal right.