NIM25029 - Class 3 NICs: time limits for paying Class 3 NICs: applying the higher rates
Section 13(6) of the Social Security Contributions and Benefits Act 1992
If Class 3 NICs are paid after the end of the second tax year after the one for which they are being paid, they should normally be paid at a higher rate. Each contribution is payable at the highest Class 3 NICs rate applicable in the period from the week the contribution was due to the date the contribution is paid. This does not apply in the circumstances described at NIM25030.
Regulation 64 of the Social Security (Contributions) Regulations 2001
If HMRC invites payment of Class 3 NICs in the last month of a tax year and they are paid within one month of the request they will remain payable at the rates which applied at the time the payment was requested.