Family Car Intermittent Non Starts (Re Occuring every month June, July, August..)

2012 Volkswagen Passat Estate TDI 2.0 Blue Motion.

Hello, I am new to the forum. I have been searching online regarding our car and very disappointed with not being able to fix intermittent non start issues. It works and then it doesn't and if I have the patience to wait 45min to 2 hours it will start again!

Any one having the same issue or am I the only one. I love it, but when I use it I am anxious it would not start when I parked somewhere away from home and then I would have to wait until it starts. No noise, no cranking, silence - but it starts again.

1) diagnostics conducted showed nothing wrong with the car (Arnold Clark £139Incl Vat)
2) Has new battery (replaced by mechanic - battery was not the issue) (Tried another Garage, mechanic thought it was the battery - £385)
3) ECU component sent for testing at ECUTesting.Com - Found no fault (mechanic adamant this was the issue £180 + £75)
4) Booked at another Arnold Clark garage who has a Master Tech, Local Arnold Clark has not got one but suggested to get a new used car)

Look forward to hearing from some of the forum members!

Could be immobiliser, starter motor, crankshaft sensor or so many other potential causes. 
It really does need specialist diagnosis rather than some of the guesswork that has been carried out so far.
Granted you have to start somewhere but after considerable expense and still not sorting the issue I can imagine you are rather disappointed.
I would try to work out when the issue occurs: hot, cold, wet, dry, near a mast (it does happen with EMF swamping ECU's), clutch not depressed enough - just simple checks to eliminate the obvious