Generally the glow plugs don't have much effect on the starting of Hdi's although its worth a try. They need to turn over pretty smartly so a good battery is needed. If its done any ammount of miles the problem is likely to be low rail pressure or the pressure sensor on the rail.
I have a C5 with the same problem at the moment, I have changed the sensor without improvement so have a replacement pump on the way, they can be obtained secondhand quite reasonably on ebay, a new one costs a fortune.
Check also that the lift pump in the tank is working, you should hear ir run briefly when you turn the ignition on, you can also check by disconnecting one of the filter pipes and briefly turning the ignition on.
Edited by Peter.N. on 05/12/2012 at 12:33
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