Hi there
I've not presently got time to write a full and specific response, so really just bumping this up the list of threads so that someone more in the know may come along and give some advice.
Sounds like you need to inform the bank and debt agency asap that the account and supposed sums owed are in dispute.
Hopefully someone will point you in the right direction and give you an idea of a letter.
Make sure you send all letters to all by recorded delivery, and do not deal with any of them over the phone.
In the meantime have you had a read of this thread ?
Claiming on a Business account? Lets join forces?
It's very long I'm afraid, but there are some very useful bits in there, and maybe some stuff that can help you.
It may also be worth considering involving the Financial Ombudsman Service (FOS), and informing of your situation and how it has arisen.
Good luck and best regards
PM