Renault master 2008 2.5 dci won't start

Hi, I have searched the net for months now for a soloution to this problem, I notice it is quite common, with no definitive answer! There is a lot of very helpful replies to similar symptoms. Let's start with the symptoms, the engine fires briefly then drops out, if you keep the starter motor engaged it will again fire up for a few seconds then drop out this cycle continues as long as the starter motor is engaged. I had the diagnostic guy out to read the codes which indicated water in fuel and air flow meter issue. Drained and fitted new filter, new air flow meter, no change. He said all waveforms are ok,fuel pressure etc., I have replaced cam sensor as well. This van has been stood a few months now as we bought another master same year 2008, which has now failed due to head gasket issue. So took this opportunity knowing I have a supply of known working parts to try! I replaced the common rail, and fuel pump ,egr valve, crank position sensor, still no change, so I went for the ecu as a 3 part swap over ie- ecu, key and transponder,and the uch unit. Although the vans are both 2008 models I was unable to try out this last option as the uch units were different ie one a 2 port the other a 3 port! So before I obtain a ecu kit, can anyone help me out ? By the way it will run on easy start which suggests cam timing ok.

Renault master 2008 2.5 dci won't start