This particular case has lots of similarities with the case of Hillis& Co Ltd v Arcos Ltd (1932) where such an option was tried to be exercised and the offeror refused to carry out that option.
Mercurial property, in this case, is responsible to carry out with the order. The same amount of rent should be obligatory to be paid. Ascertaining the terms could be obtained with reference to previous course of dealings between the parties.