When I had the silencer box off to repaint it (described on a different thread) I left the bike outside and it got caught in a short spell of rain. When I went back out I noticed water dripping from under the big box just behind the main stand (the catalytic converter). I had noticed water dripping under the bike when I started up in the morning but thought this was just condensation seeping at the joint between the silencer and the main pipe. Now this couldn't be the case as I had the silencer off. It seems rain had run inside the slightly upturned exhaust pipe and dripped out somewhere.
With everything reassembled and the engine running I managed to find this split with my finger...
I don't know how clear it comes out on the picture but the split is very small and seems to be alongside some internal features of the Cat. It's right underneath facing the road and can only bee seen easily with a mirror or a photo on a smartphone. I took it to Ducati Manchester who commented that it would be one hell of an eagle-eyed MOT inspector who found it but, as I reasoned, it wasn't going to get any better they had no hesitation in requesting a warranty replacement from Ducati. It definitely looks like a manufacturing or stress related failure. Thus in a twinkling of an eye (no of course not, it was the customary wait we all know about) the cat was replaced and all is well.
A big thanks to Mark and his team at Ducati Manchester because I thought I might have trouble with that one. (I don't have any links to them just saying they served me well. If they ever don't, I will also complain).