A tenant may withhold payment of a service charge if—
he is required to provide it by virtue of section 21, or
The maximum amount which the tenant may withhold is an amount equal to the aggregate of—
An amount may not be withheld under this section—
If, on an application made by the landlord to the appropriate tribunal, the tribunal determines that the landlord has a reasonable excuse for a failure giving rise to the right of a tenant to withhold an amount under this section, the tenant may not withhold the amount after the determination is made.
Where a tenant withholds a service charge under this section, any provisions of the tenancy relating to non-payment or late payment of service charges do not have effect in relation to the period for which he so withholds it.