4Regulations may, in relation to Class 1 contributions, make provision—
(a)for calculating the amounts payable according to a prescribed scale or otherwise adjusting them so as to avoid fractional amounts or otherwise facilitate computation ;
(b)for requiring that the liability in respect of a payment made in a tax week, in so far as the liability depends on any conditions as to a person's age or retirement, shall be determined as at the beginning of the week or as at the end of it;
(c)for securing that liability is not avoided or reduced by a person following in the payment of earnings any practice which is abnormal for the employment in respect of which the earnings are paid ; and
(d)without prejudice to the foregoing sub-paragraph, for enabling the Secretary of State, where he is satisfied as to tne existence of any practice in respect of the payment of earnings whereby the incidence of Class 1 contributions is avoided or reduced by means of irregular or unequal payments, to give directions for securing that such contributions are payable as if that practice were not followed.