vauxhall astra 1.7ctdi losing water?

my engine seems to be losing water but no visible signs of a leak , heater blows cold when engine is idling then gets a little hotter when engine revs are up and after a couple days after topping up the expantion tank with water it seems to be dissapearing but i do not see any leaks from anywhere i have checked th oil and there does not seem to be any water in it so i dont think its the cylinder head any suggestions would be much appreciated