5U.K.The amount of the reduction to be made in respect of the payment shall be arrived at by—
(a)ascertaining the sum which bears to the excess mentioned in paragraph 4 above the same proportion that the amount mentioned in paragraph 3(b) above bears to the amount mentioned in paragraph 3(a); and
(b)deducting from that sum the total amount of any reductions which have been or fall to be made under this Schedule in respect of payments previously made under the scheme in respect of the trade.