P had agreed to lease a property to D and stated mistakenly that the rent would be negotiated by the parties, failing to state what would happen should they fail to reach agreement. D realised this mistake but deliberately did not tell P of it. When no agreement was reached, CA allowed P’s claim for rectification of the lease, stating that in the absence of agreement, an arbitrator’s decision would determine the rent. CA said that where one party to a document was aware that it did not give effect to the common intention of the parties due to a mistake on the part of the other party and executed the document without telling the other party and where (per Buckley L.J.) the mistake was one calculated to benefit him, or (per Eveleigh L.J.) the mistake was detrimental to the other party, he was precluded from resisting rectification on the ground that the mistake was not, at the time of execution of the document, mutual.