In the following provisions of this Act "service charge" means an amount payable by a tenant of a dwelling as part of or in addition to the rent—
which is payable, directly or indirectly, for services, repairs, maintenance , improvements or insurance or the landlord's costs of management, and
the whole or part of which varies or may vary according to the relevant costs.
The relevant costs are the costs or estimated costs incurred or to be incurred by or on behalf of the landlord, or a superior landlord, in connection with the matters for which the service charge is payable.
For this purpose—
"costs" includes overheads, and
costs are relevant costs in relation to a service charge whether they are incurred, or to be incurred, in the period for which the service charge is payable or in an earlier or later period.