Cat is obviously on its way out. The only way around it might be to thrash the hell out of the car before they emmisions test it. Usually you will get a fault code if its anything else although the 2nd lamda should have detected a rise in emmisions. It may only just be starting to go. New cats fitted from Ford are over £900 so look elsewhere, and ignor the sports cats, they all come with a disclaimer that they may not lower emmisions enough to pass an MOT and most dont.