In any year a council of a region or islands area may defray, out of the regional or, as the case may be, the general rate, such part, as they think fit, of the expenditure incurred by them in meeting any requisition under Part IV or VIII or in performing any of their functions under any enactment in relation to water supply in their region or area in so far as not otherwise met, to an amount which, unless the Secretary of State otherwise approves, shall not exceed one-third of the said expenditure.