You should start by getting in touch with the HR department at the company that employed you, or with the financial institution that sent you reports on your PAYSOP plan assets. If you have a statement, call that company to find out how you can access your plan assets. Assuming your employer or its successor through sale, merger, etc. is still in business, they should still be overseeing your plan.
It sounds like this is probably a fairly old plan because PAYSOP plans were repealed by the Tax Reform Act of 1986, though deposits may not have stopped in 1986. If the company went out of business, and the account held company shares as PAYSOPs generally did, then there is probably no account value left and it may have been closed out.
If you have even an old statement, an advisor should be able to help you track down the successor to that institution, which could be the current recordkeeper/custodian. You can also try https://www.unclaimed.org/ as a last resort.