On both the 8v and 16v the pipe is 55mm when it exits the cat, on an 8v this reduces to 50mm between the cat and the middle box whereas the 16v remains at 55mm.
If you have the system from the cat back including the pipe to the middle box it will fit without a reducer.

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Dom
I've checked the middle section and this includes both the short pipe that connects it to the back box and the long pipe from the middle section that links to the front of the car (I'm guessing this is what will ultimately attach to the cat). From my limited research on the web I think the system I've bought would be clased as 'Cat Back'.
So based on your advice above it seems that this wll be a direct fit.
Fingers crossed.
Tom