21.—(1) If the relevant income in a particular year does not exceed £14,821, a travel grant must be of an amount equal to the school travel expenditure to which it relates.
(2) In any other case, the travel grant must be of the amount, if any, by which the school travel expenditure to which it relates exceeds the amount (rounded down to the nearest multiple of £3) equal to one-twelfth of that part of the relevant income which exceeds £14,583.
(3) Travel grants may be paid in the course of, or immediately before, a school year, on account of the grant which it appears to the school will be payable for that year.
(4) Any overpayment or underpayment of travel grant for a particular school year must be adjusted by payments between the applicant and the school within three months of the final determination of the amount.