42. Taxable amount for certain supplies.
(a) Subject to paragraph (c), the amount on which tax is chargeable in relation to a supply of goods referred to in section 19(1)(e)(ii), (f) or (g) or a supply of services by virtue of regulations made for the purposes of section 27(1)(a) or (b) shall be the cost (excluding tax) of the goods to the person supplying or acquiring the goods or the cost (excluding tax) of supplying the services, as the case may be.
(b) The amount on which tax is chargeable in relation to a supply of services by virtue of regulations made for the purposes of section 27(1)(c) shall be the open market price of the services supplied.