Finding old pensions - surely NI should suffice?

LHW99

That includes - as every pension scheme in land pleads on just about
every communication they send to members - keeping your contact details
up to date when you move, get married and so on.

You can only try. Unfortunately, One I had that had changed hands since the 70's and which had originally been sent and acknowledged a change of name document hadn't handed it on and the new company requested it again when it was claimed (all £100 of it!)