UPDATE
Up to now, given it gives me little to no bother, i have just left it like this, and ran the car as is. But recently it overheated - cold heaters, steam out back, temp gauge suddenly at top - so i stopped and let it cool down, but the water level never got beneath a quarter on the tank. i assumed the cap to be at fault and got a new one, its been fine for about 150 miles, not lost a drop of coolant, then today again the same thing, after about an hour and a half of spirited driving in the country mixed with slower town driving, only this time heaters went cold, temp gauge went up to 3/4, so pulled in and turned off, the expansion tank was full (it was half full at the start of the journey) and no steaming. After 15 mins or so ive driven 5 miles home, its then on half way again (and drove home perfect, red hot heaters, one notch under half way on the gauge etc).
Very confused, how can i temporarily gain coolant, why suddenly is it doing this on 2 different caps, surely i dont have this badder luck with caps, any ideas? or is this a nod at me being in denial about hgf?
Thanks, James