How do I get bank to confirm that I worked for them for 12 years prior to 1996?

I worked for the Bank of Scotland (HBOS)for 12 years from 1984-1996. I now have a new employer who will increase my salary because of my past working experience. When I contacted the Bank they said that they do now keep records prior to 1996. How can I get them to acknowledge my 12 years service?