1A notice given by a qualifying tenant of a flat under section 42 shall be of no effect if it is given—
(a)after the tenant has given notice terminating the lease of the flat (other than a notice that has been superseded by the grant, express or implied, of a new tenancy); or
(b)during the subsistence of an agreement for the grant to the tenant of a future tenancy of the flat, where the agreement is one to which paragraph 17 of Schedule 10 to the [1989 c. 42.] Local Government and Housing Act 1989 applies.