Income and Corporation Taxes Act 1988

4 In the absence of a requirement or election that paragraph 5 below should apply, the amount of interest at the official rate payable on a loan for any year (“the relevant year”) shall be ascertained as follows—

(a) take half the aggregate of—

(i) the maximum amount of the loan outstanding on 5th April preceding the relevant year or, if it was made in that year, on the date on which it was made, and

(ii) the maximum amount of the loan outstanding on 5th April in the relevant year or, if the loan was discharged in that year, the date of discharge;

(b) multiply that figure by the number of whole months during which the loan was outstanding in that year, and divide by 12;

(c) multiply the result by the official rate of interest in force during the period when the loan was outstanding in that year or, if the official rate changed during that period, the average rate during that period ascertained by reference to the number of days in the period and the number of days for which each rate was in force.

For the purposes of this paragraph, months begin on the sixth day of the calendar month.