Where section 45(4) of this Act applies and for any statutory period there is with respect to—
the provision of services for the tenant by the landlord or a superior landlord, or
the use of furniture by the tenant,
or any circumstances relating thereto any difference, in comparison with the last contractual period, such as to affect the amount of the rent which it is reasonable to charge, the recoverable rent for the statutory period shall be increased or decreased by an appropriate amount.
Any question whether, or by what amount, the recoverable rent for any period is increased or decreased by virtue of this section shall be determined by agreement in writing between the landlord and the tenant or by the county court; and any such determination—
may be made so as to relate to past statutory periods; and
shall have effect with respect to statutory periods subsequent to the periods to which it relates until revoked or varied by any such agreement as is referred to in this subsection or by the county court.