Well, as always, that didn't go to plan at all did it, the centre section slid off the cat beautifully, and then...
The 3 bolts holding the rear section to the centre section predictably wouldn't undo, one is unreachable with a grinder due to the fuel tank, and even with the car as high as it will go on axle stands there wasn't enough height available to twist the 2 sections together into a position where I could get a grinder in on the 3rd bolt, so I cut my losses and chopped the rear box off as close as possible to the silencer for welding based reattachment later on.
Once the box was off it was evident that whatever is in there is goosed as it sounds like it's full of gravel when you shake it, so I decided to use the stainless item I mentioned earlier, but first it was time to clean and reassemble the front joint after chopping the 3 bolts and splitting the joint, good old grinder chopped through the bolts in seconds.
Evidently the middle section wasn't ever fitted as it should be previously as the joint was only pushed together barely an inch due to the mark on it, not even half way down the flare, lots of paste, precision rubber hammery and new clip on, all good, not a hint of a leak and due to the centre section being an inch further forward now it actually fits the floor of the car better now.
The hanging points on the aforementioned stainless box were wrong, so I chopped those off the old back box and fitted everything loosely, then grabbed the trusty mig so I could tack the hangers into place, then take it off the car and weld it all up properly, before another test fit and tack the pipework side of things to again remove, weld properly and then the final fit, well, I got the hangers tacked and one of them welded fully, then the welder started spitting and being rubbish, gas has run out.
I'll shoot to the motor factors sans rear box for some more and complete in the morning.