7. The right to buy does not arise if the qualifying dwelling-house has features which are substantially different from those of ordinary dwelling-houses and are designed to make it suitable for occupation by physically disabled persons, and—
(a)it is one of a group of dwelling-houses which it is the practice of the landlord to let for occupation by physically disabled persons, and
(b)a social service or special facilities are provided in close proximity to the group of dwelling-houses wholly or partly for the purpose of assisting those persons.