I thought about auto-shutdown, but it's a more precise point of cutting out than a potential random electrical drop, plus the LED/signal test I've now done with the fuel solenoid I believe eliminates anything about the ECU shutting the engine down as it doesn't try to kill the fuel supply/solenoid (Which would be an obvious first shutdown kill-switch to pull).
One thing, on the .com forum somebody mentioned the solenoid is more a valve than on/off, so a binary LED test may not be 100% accurate, although it did stay constantly on, so it was good enough for my test.
Nev, not sure on the fuel line block, not sure how to check that easily either, other than somehow cutting a bit of line and putting clear pipe to 'watch' for bubbles or collapse of the pipe. High pressure fuel pump under-delivering is a real possibility. The HP pump is directly driven from the timing belt, unsure if it's 'timed' as such on these engines and if that has an impact, but my understanding is the HP pump gives out a constant 'high' pressure fed to the line, the injectors take their pressure/required squirt and anything over is returned via the over-pressure/return lines, so in theory if the pump isn't giving out enough pressure it would affect all engine speeds, not just higher.
True enough on not being able to re-pressure the system, Steve and my own experiences show the low-pressure/electric lift pump seems to HAVE to run to re-prime the system which fits with your experience on the Galaxy.
I'm wondering if a punt of a second hand injector and then doing injector swapping might highlight one or more faulty ones causing it. Long shot, but again so far I've spent very little cost on this so a £50 ebay injector might be worth a go, providing they're not wedged in too tightly.
Steve, mine also developed this, have had it almost 3yrs now and other than occasional tough starting in the cold (even after glo plugs changed) it's never failed in this way, so yes I'd agree it's developed and got worse over time, so wear and tear probably.
Also Steve, good luck on the fuel filter replacement, it seems a lot of people after doing that resolve this problem, so I think there are a few different scenarios that cause this issue, so there may not be one right answer unfortunately. I'm tempted to change the filter on mine, it looked fine to me and was replaced last year but for the sake of another 20 mins and £30 it might be worth doing just incase something silly like a load of gunk in there has jammed it that I can't see and is choking it (Kinda contrary to my thoughts earlier on the HP pump, but at the moment I'm trying just about anything in reason!)
As always will keep posting back :-) Good luck Steve, thanks for posting back it's appreciated having another set of eyes on the problem. Pity you're all the way in sunny France otherwise would happily swap notes over a cuppa!