how to change glow plugs for vauxhall zafira diesel 2.0 litre?

i am having a problem at the moment with my zafira. after it was dianosed, the main dealer said that i have to change my glow plugs and timer, egr valve and temperature relay. the car was loosing power from the engine and the warning lights comes on and stay for a couple of minutes. i have paid 250 to diagnose and to fix the problem. but unfortunately it was not mended properly. i nearly had an accident due to this problems. and the dealer was asking for another 690 to replace all this parts for my car. can somebody help me with this please, i will appreciate the help in deed.