Citroen Deisel prob

[Skyedriver]

Hmmmm, work colleague has a C2 with a 1200 turbo deisel engine. Runs perfectly once started but leave it overnight and it starts and stalls immediately. There is a hand primer under the bonnet and primed with fuel again it starts and runs. Prob may be solved by parking face down a hill or this may have been coincidence. My own thought is that there must be some sort of non return valve which stops fuel syphoning back to the tank and this must have failed or maybe the tank breather has become blocked, but modern deisels etc confuse me with all their electronics and stuff. Anyone any ideas what it might be, the Citroen garage cannot find it on their diagnostics but they are probably trying it with the engine running.
Cheers and thanks for any guidance
Tony H