ChD 6 October 1986. Plaintiff landlords sued defendant tenants for an alleged deficiency in the ground rent tendered for a site on which the tenants had constructed an extension to its headquarters. The tenants counterclaimed for rectification of a lease executed in 1980 between the parties in respect of the site. Both parties believed that a rent review formula expressed in a lease was to the same effect as a formula expressed verbally in the agreement to grant the lease, when in fact it was not. The formula in the agreement was more favourable to the tenant than the surveyors` formula in the lease. After a rent arbitration the landlords claimed the higher rent after applying the lease formula while the tenants tendered the lower rent produced by the agreement formula. The landlord issued a writ claiming the difference; the tenant counterclaimed. Judge held that the remedy of rectification required a mistake in the effect of the document which it sought to rectify. In this case the