1997 2.5d transit wont start?

The started motor turns the engine over fast but engine doesn't start, after pushing it or rolling down a hill and bump starting it. runs fine for the rest of the day, even if you turn the engine off straight after bumping it starts almost without urning over at all!

I have parked it up for over 5hours and it still starts fine, just over night it has never started.

any Ideas would be amazing