12.—Any payments made to a District Court clerk under an attachment of earnings order shall, when transmitted by him to the person entitled to receive those payments F50[or, where appropriate, to the competent authority], be deemed to be payments made by the maintenance debtor so as to discharge—

(a) firstly, any sums payable under the relevant antecedent order, and

(b) secondly, any costs in proceedings relating to the relevant antecedent order payable by the maintenance debtor when the attachment of earnings order was made or last varied.