5. The amounts specified in any statement under paragraph 3 for the instalments payable in the year or part of a year to which the statement relates shall (apart from any rebate under Article 28) be equal, except that the Department may round off the amount of any of the instalments other than either the first or the last to the nearest pound and adjust the amount of the first or, as the case may be, last of those instalments accordingly; but the Department may by a further statement in writing make such adjustments in those amounts as may be necessary in consequence of any change in the amount the occupier is liable to pay by way of rates in respect of that year or part of a year.