We know it can be tricky to keep track of old pensions and who they are held with. The best way to track these down is often the government tracing service. This asks for your employer at the time, and searches to see who the provider of their pensions was/is.
If you don’t know your policy number, the best course of action would be to search your emails or any old letters from the provider. Failing that, you can usually give them a call to recover this.
If you are struggling with either of the above, please drop the team a message and they’ll see what they can do to help!