Hi there, I may not have an answer to your question, but hopefully have some answers (or opinions !) to exhaust questions.
I've also got a '97 'R' and my centre section is going.
£420 is absurd. It should be more like £200. (I assume that it was the exhaust only and not the cat as well ?)
Please please, don't EVER buy an exhaust (or anything else for that matter) from Kwik Fit. Their products can best be described as absolute rubbish.
As far as I know, the early 155s had a 'sleeve' mount from the cat-exhaust. The later ones had a 3 bolt 'flange' type - which are much easier when it comes to changing.
If you still need an exhaust, please email me as I have a friend how works for an Italian car specialist and may be able to get you an exhaust at a reduced price.
I'm in London, whereabouts are you?
Also, the standard 155 exhaust comes in one complete unit (centre section and back box.)
After-market systems sell them as separate 'centre' and ‘back box' units. If you buy just a back box from say Supersprint, you will have to cut your old back box off (literally, with a hacksaw) and then sleeve fit the Supersprint one on (which is fairly easy.)
Remus also does a back box (£240 or thereabouts) which has a nice twin 3" tail pipe. Remus offers a lifetime guarantee on their products and in my opinion are exceptional build quality and awesome, but subtle noise.
Hope this help!
Stay away from Kwik Fit !