(1) Where, subsequent to the issue of an invoice by a person to another person in accordance with section 66(1), the consideration as stated in that invoice is increased or reduced, or a discount is allowed, whichever of the following provisions is appropriate shall have effect:

(a) if the consideration is increased, the person shall issue to that other person another invoice in such form and containing such particulars as may be specified by regulations in respect of the increase;

(b) if the consideration is reduced or a discount is allowed -

(i) the person shall issue to that other person a document (in this Act referred to as a "credit note") containing particulars of the reduction or discount in such form and containing such other particulars as may be specified by regulations, and

(ii) if that other person is an accountable person, the amount which the accountable person may deduct under Chapter 1 of Part 8 shall, in accordance with regulations, be reduced by the amount of tax shown on that credit note.