I know this might sound strange, but are you sure you are not over-filling the Header Tank - The Full Mark is actually directly below the filler and is only about 2mm raised from the bottom of the tank directly below the filler, (not the other end) and to fill the Header Tank from that point takes about a litre of coolant! If you over-fill it will just push out the excess.
Basically if you can almost see all of those 2 little ridges to where they join the bottom of the Tank below the filler cap, it's full.
I hope I've explained that OK.....
