Hey buddy , sorry yer not having much luck with your van .Honestly sounds like vw haven't been able to trace the fault but because your van is 4 years old then it's just flagging up that water pump and belts need done ,I would honestly look for a independent vw specialist and not go to vw directly especially if you are out of warranty ,just my opinion as from my experience independent garages have alot more knowledge than main dealers and are cheeper , it sounded like once you had sorted the charging issue with you van that the Glo plugs were getn sufficient power to work correctly and that fault had cleared , could be long shot as it's a different vehicle but my friends T5 battery was on its way out and he was having starting issues and turned out there was not enough power in the battery to heat the Glo plugs correctly , is the van still starting correctly and has the battery been tested ?