For the purposes of the covenant mentioned in section 155(2), the landlord may demand such sum as he considers appropriate, up to and including the maximum amount specified in this section.
The maximum amount which may be demanded by the landlord is a percentage of the price or premium paid for the first relevant disposal which is equal to the discount to which the secure tenant was entitled, where the discount is expressed as a percentage of the value which under section 127 was taken as the value of the dwelling-house at the relevant time.
But for each complete year which has elapsed after the conveyance or grant and before the disposal the maximum amount which may be demanded by the landlord is reduced by one-fifth.
This section is subject to section 155C.