What is the test for determining whether a contract can be specifically performed?

In Radford v. De Froberville, [1977] 1 W.L.R. 1262, [1978] 1 All E.R. 33, the contract could not be specifically performed. Oliver J. put the question in this form at pp. 56-57: … at what date could the plaintiff reasonably have been expected to mitigate the damages by seeking an alternative to performance of the contractual obligation? … In such a case, the plaintiffs right to damages must be qualified by his duty to mitigate. The question then … comes down to one of the reasonableness of the steps actually taken by the plaintiff and, in my judgment, the proper approach is to assess the damages at the date of the hearing unless it can be said that the plaintiff ought reasonably to have mitigated by seeking an alternative performance at an earlier date, in which event the appropriate measure would seem to me to be the cost of the alternative performance at that date.