What I wrote here was true to the best of my knowledge when I wrote it. However on closer inspection of the lights on the dashboard I have realised that the normal sequence when you turn on the ignition is for the immobiliser light (a little orange padlock symbol) to flash once and then go out. If this does not happen and the light stays on, then when you turn the engine over the van won't start. To rectify the problem all you have to do is turn the ignition off and back on once or twice and it reverts to the normal sequence of the immobiliser light flashing once and then going out. When the correct sequence has occurred the van always starts first time. I am absolutely kicking myself that I have not spotted this before. I'm also concerned that my local garage didn't spot it when I asked them to look into the intermittent non starting on at least two occasions. I guess there must be a glitch in the immobiliser software which is causing this and no doubt a Fiat Professional dealer could re-programme the system to eliminate the glitch however I don't think I'll bother with having that done. Many thanks to all of you who contributed to this thread.