Voluntary NI contributions class two from abroad

Hello,HMRC have confirmed that I can pay back two years of class two contributions.When I went onto the payment page,it asked for an 18 digit number,I contacted HMRC,and they sent me the number,I haven’t logged into the payment page yet,but I’ve just read on this forum that the number is made up of your national insurance number,surname,and that it doesn’t have to be 18 digits,it can be shorter,but no longer,can someone enlighten me,as I’m a little confused,if I don’t need an 18 digit number,why did HMRC give me one,someone said that was required for class 3 from the UK,I’m abroad and paying class two,thank you!

Hi DouglasFir61 Doleman,
When an individual wishes to pay voluntary National Insurance contributions by bank transfer in the UK they require an 18 Digit Number.
As you are overseas your 18 Digit Number will always be the same format and will be your National Insurance number, then IC and then your surname and initial.
If this takes you over 18 Digits then please use as much of your surname as possible.
So an example of your 18 Digit Number would be:
QQ123456AICJONESA
Thank you. 

Dear HMRC: please note that your example is actually 17 digits. Can you confirm that the format is for UP TO 18 Digits (which is always going to be less than 18 Digits if your surname is 5 letters or less). Thanks!

Hi Keith,
I can confirm that the 18 Digit Number for customers living overseas is up to 18 Digits.  
The format of the 18 Digit Number is:  Your National Insurance number, IC, Surname, Initial.
I hope that this is OK.
Thank you.