ChD 9 June 1992. Sought declarations as to the proper construction of an early rent review clause in headlease based on rents derived from underleases. The lessor was to receive a rent calculated by reference to the rents achievable in the underletting of 22 individual shops. The term of the headlease was for 125 years from 31 March 1961. Clause 4(1) of the lease defined `a specified period` for the purposes of rent review as being to the first period of 28 years, four further successive periods of 21 years and a final one of 13 years. The headlease had a fixed rent for the initial period of 28 years, the lessor was to receive an additional rent calculated by aggregating the rents achieved during the last 21 years of the initial period and then taking one twenty first of that sum estimating the rents expected to be achieved during the subsequent period and dividing that by 21. However if the latter sum is greater, the landlord is to have an additional annual rent of one fifth of the