In relation to the calculation of turnover in products of any marketing authorisation or product licence holder in any calendar year in accordance with the provisions of the succeeding paragraphs of this Part of this Schedule, “manufacturers' prices” shall mean, subject to the provisions of paragraph 2, the prices charged for the products by manufacturers to wholesalers, except to the extent that the products are supplied by manufacturers direct to retailers, in which case it shall mean the prices charged for the products by the manufacturers to the retailers reduced by such sum as, in the opinion of the relevant authority, represents the difference between the prices paid by the retailers and those which could be expected to be charged by the manufacturers to wholesalers according to the practice prevailing during the period in question with regard to such products.