SmartZIP from v1.87 supports adding bank details on the fly to a record.
The process begins at the New Bank/Financial Institution dialog on the constituent record:
You press F9 to launch the search dialog and enter your sort code and account number:
SmartZIP returns the status, in this case valid bank details, along with the full branch information and address. If you click select it will add the bank relationship and if necessary the bank details (if the sort code is not already on your database):
If the account and sort code are not valid it will tell you the reason and let you decide what to do next: