I had a similar problem with my A3 (170 PD engine), after many months of head scratching and cursing Audi, it turns out one of the rubber fuel lines was perished (if i recall correctly, it was the one that goes between the metal fuel lines crossing the top of the engine and the in-line fuel temperature sensor); it was barely visible by eye unless you took the hose off and bent it, but it let in just enough air to cause fuel to drain back. Replaced all the rubber lines with off the shelf stuff from Halfords, and it's been fine ever since. Admittedly the car has a ton of other issues, but at least it starts!
