For regulation 3 of the principal regulations there shall be substituted the following:
(3) The Commissioners may refuse to permit the value of taxable supplies to be determined in accordance with a scheme if it appears to them—
(a) that the use of any particular scheme does not produce a fair and reasonable valuation during any period; or
(b) that it is necessary to do so for the protection of the revenue; or (c) that the retailer could reasonably be expected to account for tax in accordance with Regulations made under paragraph 2(1) of Schedule 7 to the Value Added Tax Act 1983.