When they tested the glow plugs did they take them out to test them or did they do them in the engine with a tester? Did they test the voltage supply to the glow plugs when the ignition is turned on and for how long do you get a voltage supply for before the relay cuts out? (do not always rely on the glow plug light on the dash) If not sure put a volt meter to the glow plugs and the negative to the battery earth to see how long the voltage goes to the them when the ignition is turned on and count how many seconds it lasts for, I bet it is longer than the dash light stays. Then try turning on the ignition and counting for the amount of seconds that the power supply goes to the glow plugs then do it again counting only half the amount of seconds then try starting the engine I bet it starts. If it does do this all the time for cold starting.
arizona kid, February 2012
