Voluntary Class 3 NI Contributions – How to Pay

pw2020

I got through to the NI office (0300 200 3500) very quickly today and got my reference number. Maybe bear in mind the phone-line closes at 3.30pm). They accepted that I had researched voluntary payments, and didn't demand that I spoke to Future Pensions first (which phone-line was not responding).
Sledgehead's breakdown of the Ref No worked for me. All I can offer on the final digit (which seems to be different for everyone) is that it is the final digit of my date of birth. 
The phone-line helper did say that HMRC are revising the website information about reference numbers, to include voluntary payers such as those here. 
I was told it may take 6 weeks to show on my NI record. Not sure when it will register on my state pension, which is due in five weeks time. 
Thanks to everyone, the posts here have been very helpful.  

t0rt0ise

If it's like mine, it'll take months. In the end I called them to get my NI record updated, which should have meant the pension being correc when starting but didn't. Another call to pensions and two months later I got the increase with back payment.

grobsmith

Bit after the event but system unchanged. Sledgehammer decode seems correct within the last digit proviso. Had to ring for a number to be sure. Quite a wait to get dealt with! The NNNNN indeed is 99999 if amount >= £1000. Last digit was “2” in my case, so I suspect it is a “checksum” digit as suggested elsewhere (Damm Error Detection code).

lj12

I was given  the same "last digit" (possibly checksum code) twice before when ringing the helpline for a reference number. The other digits still matched the Sledgehammer decode. So after failing to get through the phone line, last year I made payment using the "Sledgehammer decode" and appending the previous reference(s) for the last (18th) digit (that is 6 for me). The last digit has stayed identical for me in all my payments thus far. I waited for the payment to make its way onto my record (as seen on the web interface) for 2 months. After failing to appear, I called the helpline. The person on the other side got my reference on the phone and found my payment. The next day the payment appeared on the record.