loosing water

[lolsteve]

ok so my hdi is drinking water , drove about 100 miles and its drunk nearly 2 litres. The water level light is intermitent as well but tends to not be on if im going above 70 and sits on if I go less than 50. am I right in thinking that if theres not enough antifreeze it could be boiling off a bit while driving and if i drive faster it cools down so the water isn't needed.
Have yet to find any leaks, though car was sat on driveway for good few days and didnt seem to be loosing any then.

Any common leak points/head gasket?