Engine cooling system loosing coolant - Pressure tests cold and hot do not show any leaks but after driving 20km the coolant needs a top up and there is moisture around the water bottle. VW say they think that head gasket is leaking back into cooling system and pressure is forcing coolant out at water bottle. Had an incident of overheating in heavy traffic about 20000km ago and replaced the water pump. Coolant loss is getting worse - warning light comes on after 50km. Car has done 180000km.