Sch 6 para 2
Where— (a) the whole or part of a business carried on by a taxable person consists in supplying to a number of persons goods to be sold, whether by them or others, by retail, and (b) those persons are not taxable persons, the Commissioners may by notice in writing to the taxable person direct that the value of any such supply by him after the giving of the notice or after such later date as may be specified in the notice shall be taken to be its open market value on a sale by retail.