On or before 15th December in each year the district council shall submit to the county council for their approval a detailed estimate of the cost for the ensuing financial year of the maintenance of every highway in respect of which their maintenance power is exercisable, and on any such estimate being approved by the county council, either with or without modifications, the amount to be paid by the county council under section 42(3) of this Act is, subject to paragraph 10 below, the amount of that estimate, or of that estimate as amended by any supplementary estimate submitted to and approved by the county council, or such less sum as may have been actually expended by the district council on the highways in question during that financial year.