They certainly are. If you know you had it with a particular company, you're much better off writing to them and keeping all the payout. However:
If you don't think you've had it at all, isn't there an argument to get a claims company to check for you?
Presumably you didn't pay off the full balance every month, so you had to pay interest and fines etc? I pay off my cc every month, so I don't expect to get charged for anything :shrug:
Ah, just seen you had a mortgage.