Well seing as either a) nobody has ever done this before or (IMG:
style_emoticons/default/cool.gif) nobody can be arsed to reply, let me tell you its easy.
There are four bolts, probably rusted up so give them a good douse in anti sieze/penetrating oil/ freeze spray.
THe two front ones are 17mm and you will need a long 17mm socket. The rear are two 16mm bolts and you can use a standard socket.
Once the bolts are undone the unit can be take off the car without the need to remove any other parts.
Fitting the new cat is very straightforward. Simply offer up ( dont forget the gasket for the rear joint) and bolt up.
Simples!!!