Very easy as long as the CAT nuts are not seized! Just dont buy a pattern one if you can help it, they rasp and sound crap!
You will have toi be able to get right under the car but it can be doine on axle stands. Make sure you buy new gaskets as the old ones will be squished, depending on type you can use a coin to push the rib back out on them but they are cheap (so long as in stock!). Do not use fire putty to seal, apparently you should not use this upstream of the cat as the system gets too hot.
Hope this helps, if you need any pointers let me know, I have changed mine about 6 times due to problems with pattern parts.
The whole system has just been replaced last month so they won't be seized... and I'm changing the downpipe because of EXACTLY what you say... I blundered and got a pattern one and its hissing/rasping... I can send it back and get my money back as I've spoken to the guy but I just need to source a non-pattern one first to put in.... does anyone know if the ones from MGOC or B&Gs will hiss or are they proper MGR ones?? I'm asking breakers now if they can source a 1999+ one for me to put in...
I am not sure about the B&G ones and the MGOC ones as I never tested them, I would be wary of the stainless ones though as they are obviously pattern parts. I have an origional one now that I managed to pick up from a scrap yard, the flex section is much bigger than on the pattern exhausts. My advice would be to get one from a scrap yard that is in good nick. Peterborough has a scrappy that is breaking loads of F's at the moment, they quoted me £25 plus postage for an origional, I would be surprised if they did not have one. It is that D something salvage in fengate. They are cowboys but they have the stock! You need the one with 6 bolt holes on the manifold end connector rather than 4 on the early one.
I'm sure the one that just went on was one with 4 bolt-holes to connect to the manifold... not 6... In fact... I'm just looked at the pic on ebay of the one I bought and its definately a 4-holer... my F is a late 1999 MY2000 on a V-plate... looking at my workshop manual, they changed to the 6-hole version in 2001
The cat ones will be easy, you just need a short extension bar on the socket. The other end is a little more tricky, I used one of those swivel things on a longer extension bar, all pretty standard stuff you get in a socket set. If I remember correctly the nuts at the manifold end are 15mm. My CAT has new bolts through it not the studs so they may be a different size to yours but they are 17mm. If yours has bolts as well you may find it a little tricky to hold both sides as the shape of the cat gets in the way. Remember that the exhaust strap also bolts on there, it is sometimes tricky to get everything lined up cat>gasket>downpipe>strap so you may have to do one nut up 1st then jiggle about to get the next one in!
I've just called the Peterborough breakers and they have one for £25 + £8 p&p. They seem pretty genuine guys, so try and give em a call. The number is in this post some where, i would get it for you but im at work!!