This is what I have done so far.

First: my cylinder head had a cracked injector housing so I got it repaired. Have now reassembled it but have done some other stuff at the same time including; fitting new fuel pressure regulator and removing and blanking off the EGR valve (genuine seals and washers fitted to injectors) and replaced the injector harness.

Since doing this work it starts with difficulty, runs for a minute or so then stops. I have rechecked the timing and its spot on now.

Had to charge the batt at that point.

TODAY - have spend an hour purging the system so the pump is nice and quiet. The engine started without difficulty. It run for approx 1 min and sounded nice then cut out but did not kick. Repeated purging sequence which took around 30 mins this time. Engine started with no difficulty, runs nicely for a min or so then cuts out again, still no kicking. Fuel starvation?

If I want to restart the engine I will need to bleed it again as pump is screeching again. I checked the brand new pressure switch and I think it may have a slight leak.

Am I correct in that it must be sucking air in from somewhere...and if yes....how do I find out where?

Any ideas welcome.

might be the air bleed valve in the filter housing's rear outer connection... though this didnt use to cut out just make starting issues...and the system can't suck air unless there's no fuel pressure, it will leak cos it's under pressure(if it is)

you must try with a known good pump to rule it completely out...or addapt a fitting to a gauge as to screw it in instead of the temp sensor in the FPR and watch the pressure...it must be 4 bar all the time...if it fails at least you know the answer...and as long as you dont plug in a tester you'll be fishing in troubled waters... maybe it's a crank signal failure and only the tester would show you that for sure