Where a notice under section 125 has been served on a secure tenant, he shall within the period specified in subsection (2) either—
serve a written notice on the landlord stating either that he intends to pursue his claim to exercise the right to buy or that he withdraws that claim, or
serve a notice under section 144 claiming to exercise the right to acquire on rent to mortgage terms.
The period for serving a notice under subsection (1) is the period of twelve weeks beginning with whichever of the following is the later—
the service of the notice under section 125, . . .
where the tenant exercises his right to have the value of the dwelling-house determined or re-determined by the district valuer (or where the landlord exercises his right to have the value of the dwelling-house re-determined by the district valuer), the relevant event .
In subsection (2)(b) " the relevant event " means—
where a review notice was capable of being served under section 128A in relation to the determination or re-determination but no such notice was served during the period permitted by that section, the service of the notice under section 128(5) stating the effect of the determination or re-determination,
where a review notice was served under section 128A in relation to the determination or re-determination and section 128B(3) applied, the service on the tenant of the notice under section 128B(3), and
where a review notice was served under section 128A in relation to the determination or re-determination and section 128B(5) applied, the service of the notice under section 128B(7).