10Any reference in this Part of this Schedule to the gross annual value of a house to which a local authority's housing revenue account related at the end of any financial year shall be construed as a reference to the gross annual value of that house, determined under section 6 of the [1956 c. 60.] Valuation and Rating (Scotland) Act 1956, as shown in the valuation roll in operation on the first day of the financial year immediately following the financial year in question ; so however that if no gross annual value determined as aforesaid is shown for that house in that valuation roll the reference shall be construed as a reference to such value as may be estimated by the assessor to be the gross annual value, determined as aforesaid, of the house.