Where a secure tenant has served on his landlord an operative notice of delay, this section applies to any payment of rent which is made on or after the default date or, as the case may be, the date of the service of the notice and before the occurrence of any of the following events (and, if more than one event occurs, before the earliest to occur)—
Except where this section ceases to apply on a date determined under paragraph (d) or (e) of subsection (1), so much of any payment of rent to which this section applies as does not consist of—
shall be treated not only as a payment of rent but also as a payment on account by the tenant which is to be taken into account in accordance with subsection (3).
In a case where subsection (2) applies, the amount which, apart from this section, would be the purchase price or, as the case may be, the the tenant's initial payment shall be reduced by an amount equal to the aggregate of—
In subsection (3)(b) " the appropriate percentage " means 50 per cent. or such other percentage as may be prescribed.