Hi manalive,
Details of how to make your payment of voluntary National Insurance contributions may be found via:
Pay Class 2 National Insurance if you do not pay through Self Assessment.
You may use these bank details for both of your voluntary Class 2 and voluntary Class 3 contributions – you do not need to send them separately.
As you have mentioned, the 18 Digit Reference Number that you would use is:
Your National Insurance Number, followed by IC then your surname and initial.
Please use the amounts as detailed in our letter when paying any voluntary contributions.
Please allow about 8 weeks from the date you make your payment for it to be shown on your Personal Tax Account.
Thank you.