6.—(1) Where a local authority is under an obligation to make payment to a water authority under article 3, 4 or 5 above, they shall, within 14 days of the end of each calendar month in the relevant year, pay to that water authority on account of that obligation an amount calculated by reference to the appropriate formula as applied at the end of the calendar month in question.
(2) After the date when the last instalment payment under paragraph (1) above falls due, a local authority shall make further instalment payments to a water authority on account of such an obligation as is referred to in that paragraph at such intervals as may be agreed between the local authority and the water authority or, failing agreement, as may be determined by the Secretary of State.
(3) This article is subject to article 7 below.