630. Limitation of service charges: time limit on making demands.

If any of the relevant costs1 taken into account in determining the amount of any service charge2 were incurred more than 18 months before a demand for payment of the service charge is served on the tenant3, the tenant is not liable to pay so much of the service charge as reflects the costs so incurred4, unless, within the period of 18 months beginning with the date when the relevant costs in question were incurred, the tenant was notified in writing that those costs had been incurred, and that he would subsequently be required under the terms of his