1(1)For the purposes of determining whether Class 1 contributions are payable in respect of earnings paid to an earner in a given week and, if so, the amount of the contributions—
(a)except as may be provided by regulations, all earnings paid to him or for his benefit in that week in respect of one or more employed earner's employments under the same employer shall be aggregated and treated as a single payment of earnings in respect of one such employment; and
(b)earnings paid to him or for his benefit in that week by different persons in respect of different employed earner's employments shall in prescribed circumstances be aggregated and treated as a single payment of earnings in respect of one such employment.
(2)Where any single payment of earnings is made in respect of two or more employed earner's employments under different employers, liability for Class 1 contributions shall be determined by apportioning the payment to such one or more of the employers as may be prescribed, and treating a part apportioned to any employer as a separate payment of earnings by him.
(3)Where earnings are aggregated under sub-paragraph (1)(b) above, liability (if any) for the secondary contribution shall be apportioned, in such manner as may be prescribed, between the secondary Class 1 contributors concerned.