So I had the code read today. The fault was the Oxygen Sensor behind the Cat Converter. The guy at the garage was somewhat unhelpful when I asked what causes this by saying "people shouldn't mess with their cars" (mine has an aftermarket ISIS single exhaust.
He's knocked the fault off now so the engine light has gone off but he say's it may reappear.
So if it does come back on how do I go about fixing this? My car has sport cats.