Off topic mazbe. HMRC have some 'amnesty' running on voluntary NI contributions.

Usually it's only the last 6 years you can pay in, but currently it's a lot longer.
It may be worth contacting them directly to their overseas support line.
(Note: the govt. website only shows class 3 payment catch up, whereas if you are working you can pay class 2, and that involves phone calls and letters too and fro).