(1) An Authority that has required payments under section 268, 269 or 270 from the owners of any properties may—
(a) if the total amount collected is not enough to meet the costs in respect of which the payments were required, require further payments from the owners of those properties; and
(b) if the total amount collected is more than enough to meet those costs, refund the excess to the owners of those properties.
(2) The provisions of sections 268(4), 268(5), 271 and 273 apply in relation to any further payments required, as if they were payments originally required under section 268, 269 or 270, except that in the case of payments required by a notice under section 268(1)—
(a) the Authority may require further payments of not more than 20 per cent of the amount specified in that notice, if those further payments are necessary to meet the cost of the works; and
(b) the cost of those further payments must be fairly distributed over the group of properties that is to receive the service, if there is such a group.