why wont my citroen dispatch start?

hi everybody, I have a 02 citroen dispatch 2.0 hdi. its always been a bit tricky to start in the cold. i changed the glow plugs last year and it made a little difference. when I turn ignition on the light for the plugs only stays on for a second then goes out, (not sure if that's a problem since the plugs still heat up but people are saying the light should stay on till plugs have heated up.) but recently takes about 5 seconds to start. also recently its been hard to start when the engine is hot after been on a run. it would crank over like its going to start but just wouldn't get going so have to stop and try again. would have to do this a few times before it started. and now..... it just wont start at all. the engine turns ok but just wont kick in. possibly no diesel getting through?? where do I start? any help welcome please as ive not had to sort a problem like this before.
cheers