CF83 processed. How to pay mix of class 2 and class 3 (overseas resident)

Dear HMRC Team (Other knowledgeable feedback welcome and appreciated!) I am a British citizen, but have lived and worked in Canada for the last 15 years. I have completed a CF83 form, and have received a written posted response detailing the amounts I need to pay to catch up the last 18 years (2006 to 2024) of NI contributions. I have called the pension service as directed in my written response and they confirmed that paying this amount WILL increase my state pension. The annual catch up amounts are a mix of class 2 and class 3 contributions, and the letter does not say which is which (I can work it out) with some years being mixed / split class 2 and class 3. The method of payment given to me for class 2 contributions appears simple. Using the QQ123456AICJONESA example reference as shown on the website along with bank deposit details. The method of payment given to me for class 3 contributions requires an 18 digit reference number which was NOT given to me on the written response. My question is whether I can pay ALL the outstanding contributions (class 2 and class 3) using bank transfer and the (QQ123456AICJONESA example) reference. The phone advisor was not sure. Can you please advise how to pay the class 3 portion of my catch up? Thanks in advance Jonathan.

Hi Jonathan Gregory,
I can confirm that when you are paying overseas voluntary National Insurance contributions you would always use the reference number of your National Insurance Number, IC, then your surname and first initial.  If that is too much for the 18 Digits then as much of your surname as possible (ie QQ123456AICJONESA).  
You may send one payment of voluntary class 2 and class 3 contributions.
Thank you.