Years ago, I opted out of SERPS (?) which I thikn meant that all of my national insurance payments went into a pension plan.

For the last 10 years or so I have had a full pension, paid into by me and my employer. I am assuming that the two pensions are not connected.

How can I find out what has happened to the other one (the SERPS one) and see whether it's worth cashing it in or transferring to my current pension?

This is possible as my accountant found my old one in a similare situation a few years ago. His advice was to leave it there for now, although I might look into that again.