73. —
(1) Where the supplies by any retailer include both supplies of food which are zero-rated under Group 1 of Schedule 8 to the Act and supplies of food in the course of catering, he shall either—
(a)keep such records as will enable the proportion of the value of such supplies which is to be attributed to zero-rated and all other supplies to be determined to the satisfaction of the Commissioners; or
(b)where he can satisfy the Commissioners that it is impracticable to keep such records, make an estimate of the proportion of the value of such supplies which is to be attributed to zero-rated and all other supplies.
(2) Where any retailer makes an estimate in accordance with paragraph (1)(b) above, VAT shall be accounted for on the basis of that estimate; but, if at any time he has evidence, or the Commissioners are satisfied, that the estimate is no longer accurate, he shall thereupon make a further estimate in accordance with paragraph (1)(b) above and shall inform the Commissioners accordingly, and VAT shall be accounted for on the basis of such further estimate from such date as the Commissioners may direct.
(3) Where the Commissioners are not satisfied with any further estimate made under paragraph (2) above, they may determine the proportion of the value of supplies which is to be attributed to the various descriptions of supplies and VAT shall be accounted for in accordance with such determination from such date as the Commissioners shall direct.