X5 wont start

[kc5578]

I have a 2003 3.0D X5 which is driving me insane! Every couple of weeks after being parked it just wont start. Starter motor cranks but the engine wont fire up. I have disconnected fuel line while problem was current and there was alot of air in the line. I end up getting it towed to mechanics outside of business hours (by the time tow truck arrives), mechanic comes in following day and it starts 1st go. BMW have had a look at it and seem to think nothing is wrong with it because no codes come up when hooked up to diagnostic computer, I am really stuck. Has this happened to anyone else? Or does anyone have any idea what causes this to happen? Any help would be much appreciated as at the moment this pic sums up how i feel