Here's the link to the information on the UK Help Page about the proper fields. It does provide some information on the number characters in a couple of the fields.
Not being in the UK, I don't have the ability to walk through the actual process upon logging in. If the issue persists, please contact Customer Service via phone or email for further clarification through the Contract Us link.
To my knowledge, it's just a card number. It doesn't contain any information about your account number and the branch transit number. It's just like a credit card, the number represent only the card ID.