3.—(1) Unless the charging authority and chargeable person have agreed otherwise under sub-paragraphs (4) and (5), at the same time that a return is submitted under paragraph 2 the chargeable person shall pay an instalment to the charging authority to which the return is submitted of an amount equal to the amount found by deducting amount B from amount A.
(2) Amount A is the total of all amounts which are or are to become payable to the chargeable person by way of contribution under section 9 of the Act for each day which falls within the return period to which the return which has been submitted relates.
(3) Amount B is an amount equal to the relevant proportion of amount A; and “the relevant proportion” for this purpose is the proportion specified in or (as the case may be) from time to time prescribed under section 15(4) of the Act in relation to the return period.
(4) A charging authority and a chargeable person may agree that the amount the person is liable to pay to the authority in respect of a collective community charge should be paid in such manner as is provided in the agreement, rather than in accordance with sub-paragraph (1).
(5) Such an agreement may be entered into either before or after the demand notice relating to the relevant year is issued; and if it is entered into after the demand notice has been issued, it may make provision dealing with the treatment for the purposes of the agreement of any sums paid in accordance with sub-paragraph (1) before it was entered into.