NIM17120 | Class 1A National Insurance contributions: Refunding Class 1A NICs: Refunding incorrectly paid interest
From HM Revenue & Customs · National Insurance Manual
Regulation 78 of the Social Security (Contributions) Regulations 2001 (SSCR 2001) (SI 2001 No 1004)
If a person liable to pay Class 1A NICs has paid interest on late paid Class 1A NICs, that interest is repayable where:
the interest paid is found not to have been due, although the Class 1A NICs in respect of which the interest was paid are due; or
There are no legislative provisions that allow interest to be paid on interest refunded.