(1) The rent payable in relation to a premises shall be--
         (a) the rent agreed to between the landlord and the tenant as enhanced in the manner provided in Schedule I; or
         (b) the standard rent specified under section 7,
as revised under section 9.
     (2) In the case of a tenancy entered into before the commencement of this Act, the landlord may, by notice in writing to the tenant within three months from the date of such commencement, enhance the rent as specified under section 7 and the rent so enhanced, shall be payable from the date of such commencement.