Audi Q7 3.0L TDI 2008 Intermitent no start

Over the last 6 months my car has developed a very annoying intermitent fault of not starting when hot,the engine turns over fine but refuses to start up and then maybe after quite a time I give it another try and away it goes yesterday it did it again in a super market car park and after much trying it then stopped turning the engine over and I got a message on the dash saying low battery battery will l recharge when running at this point I phoned the recovery man he arrived and attatched a scanner and there were no fault codes at all he attatched a booster battery and tried the starter it turned over but did not start we waited a while and this time it fired up he was bemused that the car showed no fault codes at all.
In febuary this year the car had a lot of work done on it due to particulate filter light coming on and going inti limp mode the DPF was removed and cleaned out and a new battery was fitted amongst other work all costing £2100. A week later the DPF light came on again I went back to the garage who was supposed to be a VW AUDI specialist who started woffeling on that It might be something else and I got the impression he was lining me up for another huge bill.
I took the car somewhere else who sorted a lot of problems for a lot less money than the first "EXPERT ?" and he got rid of all the fault codes by replacing the offending parts and found that the check engine light had been blanked off. he re installed it and it now it worked as it should. I picked the car up from him the day before yesterday and the car drove very well untill I get the no start problem again yesterday.
I should say that this no start problem had started sometime before all the work was done. Any ideas anyone

What are you missing during the no start?

Spark?
Injector pulse?
Fuel Pressure?