(1) Where a debtor or hirer is liable to make to the same person payments in respect of two or more regulated agreements, he shall be entitled, on making any payment in respect of the agreements which is not sufficient to discharge the total amount then due under all the agreements, to appropriate the sum so paid by him-
(a) in or towards the satisfaction of the sum due under any one of the agreements, or
(b) in or towards the satisfaction of the sums due under any two or more of the agreements in such proportions as he thinks fit.
(2) If the debtor or hirer fails to make any such appropriation where one or more of the agreements is-
(a) a hire-purchase agreement or conditional sale agreement, or
(b) a consumer hire agreement, or
(c) an agreement in relation to which any security is provided,
the payment shall be appropriated towards the satisfaction of the sums due under the several agreements respectively in the proportions which those sums bear to one another.