“4 (2) (a) If they think it necessary for the protection of the revenue, the Commissioners may require a taxable person, as a condition of his supplying or being supplied with goods or services under a taxable supply, to give security, or further security, for the payment of any VAT that is or may become due from – (a) the taxable person, … (4) Security under sub-paragraph (2) above shall be of such amount, and shall be given in such manner as the Commissioners may determine.”