5.—(1) If as a result of the alteration of the net annual value of a dwelling-house the rates payable for the relevant rating period are varied, the amount of the rates for a rental period shall be recalculated so as to give effect to the variation; but any such recalculation shall not affect the ascertainment of the rates for any rental period beginning more than 13 weeks before the date of the service of the demand giving rise to the recalculation.
(2) In this paragraph “net annual value” means the net annual value under the Rates (Northern Ireland) Order 1977 (NI 28).