Is this a foreign currency bank account?
It's possible for the foreign currency balance (in the dashboard) to show zero but for the GBP balance (on the balance sheet) to show a value. This value equates to the currency loss/gain.
If you go to the bank settings, there's an option to "Record the currency loss/gain movement".
All this does is journal the remaining GBP value to the currency loss/gain ledger.
