To set the format of your payment file you need to edit the Employer details.
Click the employer name in the main menu and then choose "Edit Details".
Select the "Bank Account" tab.
You'll see an option labeled "Bank Payments CSV Format". Set this to "Santander Bacs File".
Now whenever you download a file for bank payments, such as after a payrun is finalised, it'll be formatted specifically for uploading to Santander Connect.