Hi M47hew,
I'm sorry to hear that you are having an issue paying off an old balance. The team will be able to help you but you will need some information from the old account. This can be an address or a contact email address that was connected to the address you held the account.
Give the team a call on 0345 454 1111 and they can then locate the account and take the payment.
Let us know how that goes.
Thanks,