SORTED!!
Well, this seems to have been one of the longest threads in history, so I thought you'd all like to know that the car appears to be fixed.
It's hard to see at exactly what point the problem was resolved. So many parts have been changed, and with the first three of four major components that were swapped, I had Ryan remap the car after each. But I did feel I was taking the mick with the amount of time I needed from Ryan, so after a while, I started getting work done without having subsequent remaps.
Recently I had a few more parts changed, which included two short plastic hoses above the fuel tank which attach to the fuel line. (There was one on the feed line and one on the return line.) Mark changed them at Pheonix and was under the impression that a previous diesel fill-up and damaged these hoses. He reckons that filling up with diesel caused more long-term ongoing problems than anyone could have anticipated, and that this was just another symptom.
Anyway, since changing the hoses and subsequently having the car remapped, it's running like a dream!
Well, it wasn't, but that's 'cause the XS Power gaskets on the turbo and the oil return line had fallen apart, and most of the boost was getting vented into the engine bay! Darryl... ehem, helped me replace the gaskets... Well, to be more accurate, he swapped them, while I adopted a supervisory role.
It's like a whole new car now and goes like poo off a shovel. Just in time for the summer.
So thanks all for sticking with me in this, and for all your help and advice. It's been a learning curve!
P.S. Big thanks to Ryan, Darryl, Matt Harwood, Terry and Mark Curnow, who have all personally devoted a lot of time and attention looking at the car.
P.P.S. And the RLTC is fixed now too! Plus, I have a lot more gauges!