Social Security Contributions and Benefits Act 1992

4 (1) Where a trade or profession is carried on by two or more persons jointly, the liability of any one of them in respect of Class 4 contributions shall arise in respect of his share of the profits or gains of that trade or profession (so far as immediately derived by him from carrying it on); and for this purpose his share shall be aggregated with his share of the profits or gains of any other trade, profession or vocation (so far as immediately derived by him from carrying it on or exercising it).

(2) Where sub-paragraph (1) above applies, the Class 4 contributions for which a person is liable in respect of the profits or gains of the trade or profession carried on jointly (aggregated, where appropriate, as mentioned in that sub-paragraph) may either be charged on him separately or (to the extent only that the liability arises in respect of the profits or gains of that partnership) be the subject of a joint assessment to contributions made in the partnership name; and sections 111 to 115 of the Act of 1988 shall apply accordingly, but substituting this paragraph for section 111.