I would be ringing them back after the long weekend and demanding to speak to a supervisor. As you said, this happened way before the 180 day carp came in, so any chance of a dispute would have been long passed. If you get no satisfaction, I would lodge a dispute with the FOS immediately. 

Was the buyer told to send the item back, or did they get the money 6 months later and get to keep it? I don't think I've heard a bigger load of carp in my life!