I bought and fitted one of these to my old 2002 S8 FE (link below / not type approved). I needed to cut away part of the bracket but otherwise it fitted up perfectly & passed MOT emissions without any issues. I only replaced the one that needed doing, in my case the OS. I would not do more than you have to.
It's not an MOT fail to have a non type approved cat fitted, provided it passes emissions - which is all that matters for any car and that one is fitted when it should be.
http://rover.ebay.co.uk/rover/0/e114...008960f754b23c