STOP PRESS..despite my earlier posts saying that mine has always smelt too, I recently thought that the exhaust sound had altered slightly, as if it was blowing somewhere. All clamps were tight but when I held the back of my hand near the cat, I could feel a distinct puffing. This was on the front of the cat where the round pipe is welded to the cat can. It looked as if the original weld, though neat, had not really merged into the can, as I could see a crack running all the way round, though it was only leaking at the top.
My local friendly garage took it off today, cleaned it up and applied a more generous weld together with a couple of short straps to reinforce.
With the Duratec engine, it seems that the only flexibility between engine and exhaust system lies in the silencer bobbins. The GDI however, has I believe a flexi section of pipe, which may be better idea.