17(1)The amount of the supplementary charge on a supply within paragraph 2 is equal to the difference between—U.K.
(a)the amount of VAT chargeable on the supply apart from this Schedule, and
(b)the amount of VAT that would be chargeable on the supply if it were subject to VAT at the rate of 17.5%.
(2)The amount of the supplementary charge on a grant of a right to goods or services within paragraph 3 is equal to the difference between—
(a)the amount of VAT chargeable on the grant of the right apart from this Schedule, and
(b)the amount of VAT that would be chargeable on the grant of the right if it were subject to VAT at the rate of 17.5%,
(but see sub-paragraph (3)).
(3)If the basic time of supply for some of those goods and services is before the date of the VAT change, sub-paragraph (2) has effect as if the references to the amount of VAT chargeable and to the amount of VAT that would be chargeable were references to the relevant proportion of each of those amounts.
(4)“The relevant proportion” is—
where—
P is so much of the consideration for the grant of the right as is attributable on a just and reasonable basis to a right to the goods and services for which the basic time of supply is on or after the date of the VAT change, and
W is the whole of the consideration for the grant of the right.