The following paragraphs are hereby inserted in subsection (3) of section 37 of the Principal Act after paragraph (a) of that subsection: "(a1) such sums as, in the opinion of the Commissioners, represent the value of services provided by the Commissioners by way of plant, machinery, vehicles, tools, stores, spare parts and other engineering equipment and supplies for the purposes of the maintenance of the said drainage works, and (a2) such sums as, in the opinion of the Commissioners, represent