Class 1 contributions

Commentary – Tax Reporter ¶1021-900

Generally

Commentary – Tax Reporter ¶487-400
Legislation/Regulatory – SI 1978/1689 Sch. 1, para. 13
HMRC Guidance – EIM40209

National Insurance contributions

Commentary – Essential Tax Companion ¶2230

Voluntary settlements

Legislation/Regulatory – SI 2002/2006 reg. 4(2A), (2B)