Mines just done the same. Firstly, my car had been sitting over the winter and this can have an effect, especially as 200 cell sport Cats only just pass the emmisions at the best of time. I had to refit the original CAT to pass the MOT. I searched on google for "Custom exhaust bulders". They told me to take the car for a hard drive before the MOT next time and get the cat up to temp, this can make a big difference. I refitted the Mugen manifold and CAT and hope that it will pass next time, I'll take it for a pre test just to check before. IF it fails again, any custom exhaust builder can replace the CAT with a new 200 cell sport cat. Think I was quoted about £250 to have it replaced.