Yet another Non starting TD5

Symptoms: Once started. will run fine, go for miles & pulling OK. Stop for a few minutes, then it will start on the button again no problem. Leave it for any more than 5 minutes, it will start immediately then after about 3-4 seconds it will start hunting then stall. Will not restart until a couple of sessions of 20 seconds cranking! (It will spin very quickly, as if almost starting but not quite.)
Once started again, will continue running and restart as long as it is not left for more than a few minutes between start attempts. Any longer than that, it will start and stall after a few seconds.
What have I tried?
Swapped fuel pump with known working one.
Replaced fuel filter and air bleed valve
Replaced injector oil seals and washers (with OEM)
Replaced injector loom with known working one
No oil in red plug
De-EGR'd
Removed cylinder head, pressure tested by specialist, no leaks detected either in head or fuel rail. (10p engine)

After all that, exactly he same.

What do I try next? any helpful suggestions out there? (Barring a can of petrol and matches!)

when it doesnt start, try the fuel purging procedure a few times, then try to start, if it starts after purge your getting air in the fuel lines, its then a case of finding where, it does sound like injector seals though even though there done

Have tried purging before trying to restart. The pump is noisy for the first couple of cycles then quiet for the remainder. It will then start after about 10 seconds of cranking.
Agreed it may be air getting in somewhere. However have also replaced the fuel lines from front to back some 6 months ago.
Re the injector washers, the head was tested with the injectors in situ. The first test did show a slight leak on one of them, all were then removed, cleaned and re- seated. Subsequent tests showed all are now sealed.
What about the fpr? Is that a possibility?