HMRC internal manual

National Insurance Manual

Class 1A National Insurance contributions: Refunding Class 1A NICs: Offsetting Class 1A NICs overpayments against unpaid Class 2 NICs

You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.

If the person liable to pay Class 1A NICs that are due for refund is self-employed, for example a partnership or sole trader, any outstanding Class 2 NICs underpayments should be identified and offset against the Class 1A NICs refund.