Thanks Mike, great Info.
I've been changing as much as I can get out from the bottom of the radiator annually. I use a 50/50 mix of MB anti-freeze, distilled water and Water Wetter.
I've always had the theory that the thing that ruins cooling systems is tap water but I can't see any way to avoid it if I flush the system.
I recently sold my wife's 18 year old 190D and it never had tap water in it. The cooling system never required repair and worked fine up to the day I sold the car.