4.—(1) This paragraph applies where, after the service of a demand notice issued with respect to a collective community charge, a charging authority sets an amount for its personal community charge for the relevant year (“the new amount”) in substitution for another amount (“the old amount”) under section 34 or 35 of the Act which differs from the old amount.
(2) The charging authority shall as soon as practicable serve a notice on the chargeable person stating the new amount.
(3) If the new amount is greater than the old amount, any difference between the amount of an instalment paid under paragraph 3 before the service of the notice under sub-paragraph (2) and which was calculated by reference to the old amount, and the amount which, in consequence of the setting of the new amount, is in fact payable under that paragraph in respect of the instalment shall (subject to the terms of any agreement under paragraph 3(4) and (5)) be due from the chargeable person to the charging authority in a single instalment on the expiry of such period (being not less than 14 days) after the day of issue of the notice served under sub-paragraph (2) as is specified in it.