the tenant has the right, on the payment of a sum, to require the terms of the lease to be varied so that the rent payable under it is reduced, and

by exercising that right the tenant acquires an interest, additional to one already held, calculated by reference to the market value of the dwelling and expressed as a percentage of that dwelling or its value (a ““share of the dwelling””).

an election was made under paragraph 3 (shared ownership lease: election for market value treatment) or paragraph 5 (shared ownership lease: election where staircasing allowed) and any tax chargeable in respect of the grant of the lease has been paid, or

immediately after the acquisition the total share of the dwelling held by the tenant does not exceed 80%.