18.—(1) In a case where the earnings paid to a woman in respect of 2 or more employments are aggregated and treated as a single payment of earnings under regulation 12(1) of the Social Security (Contributions) Regulations 1979, the employers of the woman in respect of those employments shall be treated as one for all purposes of Part V of the 1986 Act.
(2) Where 2 or more employers are treated as one under the provisions of paragraph (1), liability for statutory maternity pay payable by them to a woman shall be apportioned between them in such proportions as they may agree or, in default of agreement, in the proportions which the woman's earnings from each employment bear to the amount of the aggregated earnings.