Just wondering of anyone has come across this fault or something similar?

2002 130 TDI sport 73K.

Starts first time every time when left on a flat surface.

Starts first time every time if I park it nose down on a slope.

If there is any (even slight) upward incline it fails to start! It takes up to 5 minutes with foot to the floor cranking the engine. Very often the oil warning light comes up and the car complains and says stop! When it eventually starts, you can stop it and it starts again first time, even if it's cold / damp outside.

I'd love to say it's an option to always park it flat / facing nose down but it's not.

I figure it must be a fuelling problem from the tank to the injectors. Does that sound about right?

Thanks