You said you had a loan with Cash2DayLimited a few years ago, and you had a PIF letter with them. The phone number you listed as calling with this debt, in your first post, comes up as a number for Cash2DayLimited.
I'd be down at my bank asap reversing that charge. Don't call the jerk back. If he calls you, tell him you will be glad to fax him a copy of the PIF letter, and inform him that you are reversing the charge, as the loan is already paid off.