If any of the relevant costs taken into account in determining the amount of any service charge were incurred more than 18 months before a demand for payment of the service charge is served on the tenant, then (subject to subsection (2) ), the tenant shall not be liable to pay so much of the service charge as reflects the costs so incurred.
Subsection (1) shall not apply if, 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 lease to contribute to them by the payment of a service charge.