Priority of mortgages
4. A mortgage securing a person’s liability to make to a registered social landlord any payment required by an assured percentage covenant or equity percentage covenant shall have priority immediately after any legal charge securing an amount advanced to that person by a qualifying lending institution—
(a) for the purpose of enabling him to acquire the estate in question; or
(b) with the written consent of the social landlord, for the purpose of enabling him to carry out any improvement to the house in question.