do you have a pre heat warning light up when started that stays on for a while after the car is running?
If the light goes straight out then its not the glow plugs, the stilo will tell you if there is a problem with the plugs, i know as ive changed them and it is a right pig of a job that can easily end up in tears as the plugs are very thin and fragile.
Put some premium diesel in the car, and an additive to give the injectors a good clean, this may solve the problem and is by far the easiest solution