Try this.
https://www.gov.uk/find-lost-pension
I worked for a company for about 2 years and stayed with my second employer for the rest of my working life.
I was paying into the pension scheme of my first employer and still had a wage slip to prove it.
This service found my pension scheme but at first the trustees said I could have not had a pension with them as people had to be 21 to join.
I sent a copy of some of my wage slips to prove I had paid in.
After some fuss it was found I was due a pension of £285 per year.
The trustees wanted to transfer it into my main pension scheme but they could not take it as I would have been receiving more than two thirds of my final salary.
I have agreed to be paid once every 6 months due to the small amount.
Out of interest the trustee said he thinks I was the only employee paying into the pension scheme before I was 21.
I started paying in the day after I left school at 16.