Starting Probs m51 td

[Pikey]

Hello
Ive had this problem for a couple of months now when i park facing up an incline for a while my car doesnt want to start, it will want try at first then have to winding the starter over for about 20 secs.
Car starts and splutters until i apply revs after that runs fine
When i park facing down a incline car starts as normal.
So replaced fuel filter and this stopped for a couple of days but problem came back.
Is there a non return valve somewhere.
Any ideas?
Thanks