“Provided also that, where all the local authorities in a catchment district agree to apportion among themselves and to pay to the Catchment Board by way of annual amounts sufficient in the opinion of the Board to provide for the efficient administration of the district but not exceeding the amount that would have been provided by the levying of an administrative rate of the maximum amount authorised by this Act, the Board may accept from the local authorities, and the local authorities may pay to the Board, such annual amounts as may be due pursuant to the aforesaid agreement, notwithstanding that the amount payable by any local authority in any one year exceeds the sum that would have become due and payable to the Board by the levying in its district of the maximum administrative rate authorised by this Act.”