Once you've logged in:
Click My Details
Click on My Bank Details
Re-enter login password
This screen will show you any bank details which are already held on the system
Click Add New Account
Enter Account Number and Sort Code (Please note the Account holder name cannot be changed here)
Tick the 'Please confirm you are the account holder and the only signature required to authorise transactions on this account'
Click Verify My Details
Bank details will now be shown on the My Bank details page.