Car sometimes doesn't start after being driven earlier

[SameerM]

Hi, my 2007 A6 3.0 TDI has developed an annoying problem.
For the first trip of the day, the car starts fine and takes me wherever I want to go. However, after driving to wherever and doing whatever, upon returning to the car it sometimes refuses to start for around 30 minutes to an hour. After this time has elapsed, it then returns to being able to start normally. So I can potentially get stuck somewhere for an hour which is not ideal.
Other information;
- For the first fail, engine cranks and it cranks strongly, but engine doesn't start, and after 4-6 cranks it stops. It cranks more times than it would if the car were to start normally. For subsequent attempts to turn the engine on, it cranks fewer times before "giving up." The cranks don't get weak, they maintain the same level. I dont make a habit of trying to turn it on multiple times after the first fail.
- Around 8 months ago I put petrol in a totally empty tank, and have had a number of problems since then. I fix them as they pop up but I assume this is related to that (since all other problems have been related to that.)
- I have taken it to an independent garage to obtain fault codes, but no relevant codes appear, and the garage are struggling to figure out what it could be. They think starter motors, but since the problem has not appeared with them, it feels like they are guessing.
- I have tried to ascertain a pattern in the type of drive that seems more likely to cause this fault, but dont have a huge sample size. It might be that longer drives make it more likely to occur since 3/4 times it's happened there has been an above average amount of driving. However it also occurred once with a short drive.

Thanks in advance for any help

[SameerM]

Hi, just in case anybody reads this, the people at the garage replaced some starter relays and the crank sensor and the car appears to be working fine now.