hello. allready done the form filling, account cannot be traced, too old!

hello. i have a tsb plc passbook from 1991 showing a balance of £400+ credit, the local bank branch do not recognise the account number due to its age, i was advised to fill out british bankers assoc. dormant account form, this was sent off with a photo copy of the passbook page. i have had a reply some 4 weeks later explaining that they cannot trace the account citing that the bank only keeps records of financial transactions for a max of 10 years by law! is this correct, if so is there any further action i can take. any help appreciated.