GU 3L TurboDiesel engine will crank over but not start

I had just left home and was driving down the road when I lost all power, When I checked the dash all lights were on, as if I had just turned on the ignition keybut no power. When I tried to restart, the engine would crank over but not fire, I checked all fuses no problems, checked fuel filter that was clear, in fact every thing appear to be OK. Next step, get RAC they came out and had no joy so they arrange for transport to local Nissan dealer. We left the vehicle in the rear entry and tried to start but still nothing. Later that afternoon the service manager ring me up wanting to know what the problem is, guess what? they have hopped in and the thing started straight away. When I re-explained what had happened, waht I had checked, then what the RAC guy had checked and tried, the service manager agreed that there must be a problem, and as yet (11.30am) they are still none the wiser. One consolation I suppose is that it didn't happen on the Sunday while SWMBO and I were out with our 4WD club day trip to York via powerline track, things could have ben interesting. Do any forumites know of this happening? If so what was the cause

Mike