i have an 05 astravan and have just had the mot and service done. it was very hard to start the engine before the service and seems worse now, the garage say they tested it and they can't do anything about it.
It sounds as if you have a faulty Glow plug, or a connecting wire that is loose, they heat up the derv to assist the starting, If you intend to replace them (4) replace the lot at the same time, you do not need to bleed the system when you replace these. the job can be done within the hour.