3(1)All sums which on the relevant date are due from the debtor on account of Class 1 or Class 2 contributions under the [1975 c. 15.] Social Security Act 1975 or the [1975 c. 15.] Social Security (Northern Ireland) Act 1975 and which became due from the debtor in the twelve months next before the relevant date.
(2)All sums which on the relevant date have been assessed on and are due from the debtor on account of Class 4 contributions under either of the said Acts of 1975, being sums which—
(a)are due to the Commissioners of Inland Revenue (rather than to the Secretary of State or a Northern Ireland department) ; and
(b)are assessed on the debtor up to 5th April next before the relevant date,
but not exceeding, in the whole, any one year's assessment.