Looking at the system and the sensor locations, surely the cat is the last box, as the DPF differential pressure is taken before the last box, so will be measuring across the first.
It's not at all clear and when you look at the options for the system, there appear to be several systems available, one or two don't seem to have a cat, unless it is incorporated into the first box.
Luckily for me, the option that I have fitted (and that looks standard to the car) is the most expensive to source. I believe that one could retrofit the other type, but sensor wiring and pressure sensor pipework would need to be moved.

Looking at how my system has failed and the indications leading up to it, I believe that the cat had a blockage, either pre-existing, or caused in transit when it was shipped, it's not beyond possible that the muffler is also partially blocked and this might account for the relatively poor fuel economy and rough running.

My options are now to replace the system (one on order) or cut out the last box and weld in a straight through tube, if the last box is indeed the cat, it's a quick and dirty fix until I get the opportunity to do the whole thing.

I hate DPFs...