Okay, thanks for that idea. Something for me to add to the todo list.
The car spent a day with Mark and Matt Harwood at Phoenix today. Alas, the problem still remains.
Mark's initial suspicion was that a hose attached to the pump might be leaking fuel. He was pleased to see that this was in fact the case: there's a short fuel hose attached to the pump which appears to have an inner and an outer sleeve. Apparently, when testing, this tube was p1ssing fuel all over the place.
He replaced this hose, and they switched the fuel pump to be on 12V permanently.
But on driving the car, the problem still remains. The FPR loses over 10psi in pressure while driving, and this appears to be (although still not proven) as a result of it getting hotter. There are no wiring issues and the voltage checks were fine.
Matt and Paul think that I may have been unfortunate enough to have replaced my fuel pump with a malfunctioning new unit. Given my luck, this wouldn't surprise me.
So, I'll order yet another fuel pump and hope that that's the answer to my problems.
I'll also be spending some more money I don't have on an in-cabin fuel gauge, to help with the diagnosis process.
On a side note, it turns out that the RLTC I bought a month back isn't quite working properly, but Matt thinks he might be able to resolve this easily enough. (This has no bearing on the FPR problems; I've had the RLTC turned off as it makes the car misfire all the time!)