Electronic payment for National Insurance contributions declared on your P11D(b) return for the tax year ended 5 April 2021 must clear into the HMRC account by 22 July 2021.
Use the right payment reference when paying Class 1A National Insurance contributions
Help ensure your payment is correctly allocated by providing the correct payment reference.
Use your 13-character Accounts Office reference followed by 2113.
Adding 2113 is important because 21 tells HMRC the payment is for the tax year ended 5 April 2021, and 13 lets them know the payment is for Class 1A National Insurance contributions. The reference should have no gaps between the characters.