X plate Mk1 Focus

Had starting issues for a while

Battery is fine, glow plugs changed, cylinder head temp sender changed

Cranks for 20 - 25 seconds before it fires

Lift pump and hi pressure pump seem fine

Normally park the car facing up my sloped drive, it with engine higher than the fuel tank, as an experiment I reversed into the drive so tank higher than the engine, and hey presto starts on the button.

So my guess is the fuel is draining back into the tank - should there be a non return valve stopping this?

Any help appreciated