It is clear, however, that there are circumstances in which the courts have made exceptions, the most striking of which are cases such as Gregory v. Mighell [43] and Beer v. Bowden [44] . This exception may be expressed: Where an agreement which would otherwise be unenforceable for want of certainty or finality in an essential stipulation has been partly performed so that the intervention of the court is necessary in aid of a grant that has already taken effect, the court will strain to the utmost to supply the want of certainty even to the extent of providing a substitute machinery.
And Lord Fraser of Tullybelton adopted this statement of the exception when Sudbrook Trading went to the House of Lords [45] . With the greatest respect, I doubt whether it is right to regard Gregory v. Mighell or Beer v. Bowden as exceptions to a rule applicable where the contract lacks certainty or finality in an essential stipulation. Rather, these cases tend to show that where the express terms of a lease reveal an hiatus in the machinery for fixing the rent, the court will lean towards a construction of the lease which treats the machinery merely as a means of ascertaining what is capable of being ascertained objectively as a fair and reasonable rent and which thus avoids an hiatus in an essential stipulation. The justification for that approach to the construction of a rent clause in a lease is manifest when the lessee has been in possession prior to the failure of the contractual machinery, for the parties cannot be taken to have intended that the lessee should have been entitled to rent-free possession if the machinery should fail to fix the rent. That intention may be imputed to them (in the absence of any contrary provision in their agreement) as well before as after the tenant is put into possession where the commencement of the term is not dependent on the prior fixing of the rent. Contracts for the sale of property may stand on a different footing where an obligation to convey or transfer would not arise until the price is fixed (cf. Hall v. Busst [46] ). Even in a contract for sale, however, the House of Lords in Sudbrook Trading has now chosen to construe a contract which requires the price to be fixed by a valuer as a contract for sale at a fair value to be ascertained by the valuer or, in default, by the court. Lord Fraser of Tullybelton said [47] :
I recognise the logic of the reasoning which has led to the courts' refusing to substitute their own machinery for the machinery which has been agreed upon by the parties. But the result to which it leads is so remote from that which parties normally intend and expect, and is so inconvenient in practice, that there must in my opinion be some defect in the reasoning. I think the defect lies in construing the provisions for the mode of ascertaining the value as an essential part of the agreement. That may have been perfectly true early in the 19th century, when the valuer's profession and the rules of valuation were less well established than they are now. But at the present day these provisions are only subsidiary to the main purpose of the agreement which is for sale and purchase of the property at a fair or reasonable value. In the ordinary case parties do not make any substantial distinction between an agreement to sell at a fair value, without specifying the mode of ascertaining the value, and an agreement to sell at a value to be ascertained by valuers appointed in the way provided in these leases. The true distinction is between those cases where the mode of ascertaining the price is an essential term of the contract, and those cases where the mode of ascertainment, though indicated in the contract, is subsidiary and nonessential: see Fry on Specific Performance, 6th ed. (1921), pp. 167, 169, pars 360, 364.
1. [1983] 1 A.C., at p. 460.
2. (1811) 18 Ves. 328 [34 E.R. 341].
3. [1981] 1 W.L.R. 522n.
4. [1983] 1 A.C., at p. 484.
5. (1960) 104 C.L.R. 206, at p. 222.
6. [1983] 1 A.C., at p. 483.