How to indicate specific years of payments when re using 18 dig ref.

I gather I can re-use the 18 digit reference number I got over a year ago to pay some gaps in my NI contributions. But now I would like to pay for 3 more specific years (out of 6 still not full) so how do I indicate in my bank payment which years I am paying for? Thank you

Hi s_wil,
It's possible that your 18-digit reference number has changed.
This could be due to the amount you need to pay or if you qualify for a different Voluntary class rate.
We also need to know if you plan to make a payment so we can allocate this to the correct tax year/s.

Call National Insurance on 0300 200 3500
You can use the webchat function ‘ask HMRC online’  Ask HMRC online
You can also write in to:
PT Operations North East England
HM Revenue and Customs
BX9 1AN
When submitting your correspondence, kindly provide your National Insurance Number, full name, current address, a contact telephone number, and a brief explanation of the purpose of your letter.
Additionally, please remember to sign and date the document, even if it is printed.
After contact, we will set up instruction on your account for receiving payment, as well as confirm or update your 18-digit reference number and any changes to our payment options.
Alternatively, you might be able to make a payment through your pension forecast on your Personal Tax Account via the Government Gateway.
Please remember that not informing us about a payment or using an incorrect reference number could delay your payment allocation or cause your payment to go missing.