Martin - if it's a registered legal charge then you'll need the lender to provide a form DS1 to discharge it. They need the account number to enable them to trace their file/records to confirm that it's been paid off and can be discharged
So you should see if you/your Mother can work back through her paperwork/own records to see if you have anything that identifies the account number yourselves
You can also use our contact form to see if we have anything on record - the document submitted for registration doesn't always contain the account number but still something to check
Public guidance: dealing with land and property - GOV.UK