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HMRC internal manual

National Insurance Manual

Refunds: retrospective change of employment status from self-employed (Class 2) to employed earner (Class 1)

Regulation 51, SS(C)R 2001

If a contributor pays Class 2 NICs when Class 1 are due and the employer accepts Class 1 status, any wrongly paid NICs should be reallocated towards the primary Class 1 due.