Consumer Credit Act 1974....
"(b)so far as the claim relates to any single item to which the supplier has attached a cash price not exceeding [F2£100] or more than [F3£30,000] [F4, or]"
It does not say £100 must have been paid, but a price has been attached. Also the act says you can claim costs back for any action you take against the debtor.
Personally i'd threaten the credit card company with small claims court. A bank isn't going to attend small claims court for anything less than £500.
I hope this helps someone,