57.—The purpose of this Part is to specify the requirements for a valid termination by the landlord or tenant of a tenancy of a dwelling, whether the dwelling is—

(a) one to which this Act applies but to which Part 4 does not apply F137[(by reason of the operation of sections 3(7)(c) or 25)], or

(b) one to which both this Act and that Part applies (in which case those requirements are in addition to the requirements of that Part with regard to the termination of a Part 4 tenancy or a further Part 4 tenancy).