My ford ka is using (NOT leaking) alot of water?

Hi my ford KA endura-e engine has started to use alot of water, i do commute daily a total of 1hour on the motorway but im finding myself filling my cars water every other day. My father is trying to repair it, the water container is not burst, the pipes are not split and the thermastat appears to be fine (although we have ordered another to eliminate it) has anyone else had this problem before or does anyone know what this could be? thank you