I was checking oil, coolant, etc the other day when I noticed the header tank was nearly empty. I refilled it. I have done 2 or 3 140 to 200 mile journeys recently since 1st spotting header tank was low. Checked it again yesterday and it had gone down again. That was after another 130 mile journey. The car is running perfectly. Nothing is showing untoward. The temp gauge sits where it always, about a third of the way up. No leaks underneath or in engine bay that I can see. If anyone has any ideas what this problem might be, any advice, I would be very grateful. Thanks.