21.—(1) Where, having regard to all the circumstances of the case, the court considers it appropriate to do so, it may, in a periodical payments order, direct that—

(a) the period in respect of which payments under the order shall be made shall begin on such date before the date of the order, not being earlier than the time of the institution of the proceedings concerned for the grant of a decree of divorce, as may be specified in the order,

(b) any payments under the order in respect of a period before the date of the order be paid in one sum and before a specified date, and

(c) there be deducted from any payments referred to in paragraph (b) made to the spouse concerned an amount equal to the amount of such (if any) payments made to that spouse by the other spouse as the court may determine, being payments made during the period between the making of the order for the grant of the decree aforesaid and the institution of the proceedings aforesaid.

(2) The jurisdiction conferred on the court by subsection (1) (b) is without prejudice to the generality of section 13 (1) (c).