what causes my vauxhall zafira 2005 2.0 diesel not to start?

A few months ago my Zafira would start and then die almost immediately. I would then have to crank the car over with the key until eventually it would start. It almost the same as having an old car with a manual choke and has the power starts to drop you pull the manual choke out and more fuel gets you through it except in this day and age their no choke so I have to let it 'die' on me. Things I done to try and combat it, new fuel filter, new glow plugs, non return valve fitted, injector rings, and still no joy. If anything it has got worse, now the engine wont turn over at all and no fuel is getting through, but what is really annoying is it is so random sometimes it works sometimes it don't. any suggestion