Hi pwylung,
As you are overseas and wish to pay some voluntary National Insurance contributions to us, your 18 Digit Reference Number will be: Your National Insurance Number, followed by IC then your surname and initial.
Details of our bank details may be found via
Pay voluntary Class 3 National Insurance
When you have made your payment please allow 10 working days to pass then call our National Insurance Helpline on 0300 200 3500 (from the UK) or +44 191 203 7010 (from overseas) and they will arrange for your payment to be allocated straight away because you are over pension age.
You may wish to have your payment details to hand when you call.
Thank you.