they do a emisions test at any mot station for cataliyst some times it will show up a fault. but the easyiest way is to take it off and see if it has collapsed if not try and run it up with out it and see if it still has the same problem. but i bet it is the cat that is at fault, or check the intake hose between the air mass meter and the intake as these tend to split causeing rough running and missing and the problems you describe