Vehicle not starting properly

I have a 3L diesel Renault Mascot the other day it was showing on the dash light that there was water in the fuel filter and the vehicle would not start. I changed the fuel filter started the vehicle and it started fine. The next morning I went to the vehicle to see if it was still okay vehicle would not start. I got it started with some PC start then it seems to be okay for a few hours but then leaving it for quite sometime it's very difficult to start again. Could there be a problem with a non-return valve? The fuel pump seems to be building up for sufficient pressure but it just seems to be starved undercoat diesel. I'm trying to start as soon as I've squirting some Easy Start if I straight up and then it just runs fine.