Hi,
There are 2 catalytic converters (cats) in the car one just after the turbo and one under the car. Removing these and replacing with pipe will take away the restriction coursed by the cats and allow the car to sound better and make more power. For best results you should get the car remapped but I felt a difference just having the second cat removed.
You need one 2 pass an MOT. Removing the second one under the car will still let you pass an MOT and improve the car.
The second cat is the one under the car and this is the one we remove and replace with the de-cat.
If you have axel stands or drive on ramps you should be able to fit the de-cat yourself but with all mods if your not confident or dont have tools needed it could be easier to get a local garage to do it for you.
To fit the decat you need to undo the bolts that hold the current cat in place and the screw in sensor. Then put the de-cat in its place and do the bolts up.