One stage further. and the two issues.
The P0089 fuel pressure code and engine light gets more odd each time. Last night, as reported, the light was off when my wife drove to her concert. When she went to come home the light was off in the car park and on leaving the event, but came on soon afterwards. This morning I went out for fuel and, as expected, the engine light was on because I hadn't cleared it. Then a quarter of a mile up the road it went off of its own accord. So who knows what is going on.
The second issue with the fuel levels is a bit further on towards a solution, but has got a bit more confusing. I disconnected the ECU and measured the resistance to each of the two level sensors at the ECU connector. The WIS says they should be 40 Ω on full and 250 Ω on empty.
I measured with the gauge showing about ⅓ full and the resistances were...
Level on fuel pump sensor: 158 Ω
Level on second separate sensor: 121Ω
Which seems about right.
After I filled the tank, I checked the resistance again and they were...
Level on fuel pump sensor: around 23 Ω (It was wandering about from 21 Ω to 25 Ω, not steady)
Level on second separate sensor: 42.4 Ω which is about right for a full tank.
So the fault seems to lie with the sensor attached to the pump. It can't be a broken wire in the resistance strip on the sensor as the reading is not zero, and it can't be a corroded connector in the line because that would give a higher resistance. But it's reading below 40 Ω, so presumably the ECU is seeing this as out of range. It implies that the float is going higher than expected, relative to the resistance winding. All I can think of is that the sensor has become detached and is loose and moving about a bit. But at least it's likely to have identified the sensor that's at fault, I think
It looks like a garage job to fix it. To get to the tank, it's:
Anti-roll bar off
Exhaust off
Propshaft off
Heat shield off
Rear diff unbolted and hung on a strap to lower it a few inches
Detach fuel filler
Remove fuel pipes
Undo main electrical connector
Unbolt tank and fiddle it out
That's too much for me these days. I might even leave it as it is for a while. Oh! for a hole in the floor like the 9-5.