“2) To pay to the lessee in addition to the rent hereby reserved: (a) the annual sum of ONE HUNDRED AND FIFTY POUNDS or such other sum that the lessor may decide in accordance with clause 3(e) of this Fourth Schedule (“the maintenance charge”) as a contribution on account towards the expenditure incurred by the Lessor in carrying out its obligations under the Sixth Schedule hereof…. (c) If the expenditure incurred by the Lessor in any accounting period of twelve months in carrying out its obligations under the Sixth Schedule hereof (hereinafter called “the annual cost”) exceeds the aggregate amount payable (or deemed to be payable) on account as aforesaid by the Lessees of all flats in the Building in the accounting period in question (hereinafter called “the annual contribution”) together with any unexpended surplus as hereinafter mentioned and a certificate of the amount by which the annual cost exceeds the total of the annual contribution and any such unexpended surplus be served upon the Lessee by the Lessor or its Agents with audited accounts in support thereof then the lessee shall pay the Lessor within twenty-eight days of the service of such certificate (which said certificate shall as respects the matters therein contained bind the lessor and Lessee unless some manifest error shall be found therein or in such accounts in which case such error shall be rectified) a proportionate part (hereinafter called “the excess contribution”) determined by the Lessor or the Surveyor of the lessor of the amount of such excess shown therein such sum to be recoverable from the Lessee in case of default as if the same were rent in arrear PROVIDED that if in any such accounting period as aforesaid the annual cost is than the annual contribution the difference (being the unexpended surplus) shall be accumulated by the Lessor and be applied in or towards the annual cost in the next succeeding or future accounting period or periods as aforesaid.”