I'd like to edit the subject of this post to include the word "solved" but I can't find an option to do so. Can a post only be edited once? Here's the latest result, at least as far as the performance issue is concerned:
After investigating a whole bunch of other stuff, including several red herrings, and after discussions here, I went out and purchased several jerry cans and borrowed a few more. I was able to empty the fuel tank and take it out of the car (a bugger of a job!). While I had it out I made some changes to where the rear strap hooks in on the outboard side. It will now pop back in relatively easily and without a lot of bending of the strap, head bumping, swearing, etc.
What I found was some kind of gunk that had the consistency of light grease. I won't try to guess what it is or where it came from, although I suspect it was in the fuel when I refilled the tank a second time after installing the tank because it was not too long after that that the car started mucking up. That gunk wasn't actually blocking the fuel pickup but, as far as I was able to determine, had worked its way up the line. The was no sign of it at the filter end, so I think it built up somewhere between the tank and filter and created a partial blockage. One which only had an effect when I had to put the foot down a bit going uphill, and therefore needed a bit more fuel flow. Anyway, while I had the two ends disconnected I fired up the compressor and blasted air through the line at the compressor's full 145PSI. I can't tell you what came out because I was on my own and can't see both ends of the line at the same time. Besides, with the pressure and volume of air that I used, I'm sure the gunk would have just sprayed everywhere and become indistinguishable from the fuel I had managed to spill all over the place.
My theory for the black smoke when the car lost power is that it initially starved of fuel. I then eased off the pedal and worked at finding a point where I still had enough power to go forward, but of course much slower than desired. At the same time, the computer was asking for the additional fuel and, because the engine was no longer using it as fast, I got a surge when the fuel caught up, creating a temporary over-fueling situation. While working to pedal to keep the car moving I think it was oscillating between being starved of fuel and being flooded with it. I am of course quite happy to be proven wrong about this if anyone has a better and convincing explanation.
About the lift pump I mentioned previously, I tested it and found it wasn't doing anywhere near enough to warrant keeping it. That, and comments posted here, made me suspicious about it being an OEM part or not, so I followed the wiring back and found the point where that had been modified. A very professional job indeed but I can't help but wonder why it was ever done if Navaras don't have a lift pump as standard. The absolutely most confusing thing for me was that the ODBII scanner was able to read the pump current. That can only happen if there is a suitable electrical connection point for the pump. I assume the Navara has support for such a pump but Nissan decided not to fit them. Anyway, that pump has been binned, the pickup assembly restored to stock condition and the float arm adjusted to give me an improved reading when the tank is near empty.
The car is now running very strong through the local hills, which have a few long steepish climbs that would most definitely have triggered the problem before. I'll repeat the testing with the camper attached but, for now, I feel confident enough to call the diesel specialists and cancel my booking with them at the end of the month.
