Where a notice under section 146 has been served on a secure tenant, he shall within the period specified in subsection (2) serve a written notice on the landlord stating either—
that he intends to pursue his claim to exercise the right to acquire on rent to mortgage terms and the amount of the initial payment which he proposes to make, or
that he withdraws that claim and intends to pursue his claim to exercise the right to buy, or
that he withdraws both of those claims.
The period for serving a notice under subsection (1) is the period of twelve weeks beginning with the service of the notice under section 146.
The amount stated in a notice under subsection (1)(a)—
shall not be less than the minimum initial payment and not more than the maximum initial payment, and
may be varied at any time by notice in writing served on the landlord.