I did mine 2 weeks ago.
You might be having a hard time as i had with removing the old one as all bolts weird seized as well.
I got a dremel tool with me and cut all the bolts myself and that did the trick.
Little fact for you to know: The bolts on the cat's are like wheel studs and are pressed in the flange of the cat, the Invidia exhaust does come with a hardware kit but it will not supply you with new bolts and gasket for this.
So before you start, get a set of gaskets that sits between the cat's and the rest of the exhaust and 4 sets of bolt, nut and washers for this.
I went to a hardware shop and bought 4 bolts the same size as the ones supplied with the exhaust (believe it was M10x40 mm)
If you lift one side of the car with a jack and support it with jack stands you will get enough clearance to work underneath it, unless your car is lowered.
AND, don't make the same mistake i did; the pipe's connected to the cat's need to be installed with the small mufflers facing the rear of the car!