If your thermostate is leaking then generally they only leak when the car is hot and the thermostat open so maybe it would not show up. From what your telling me i very much doubt your headgasket has gone. Is the car overheating?
ST170's losing coolant is quite common and 9 times of ouf 10 the reason is as Rich says the thermostate. If it is dont be told that the whole thermostate needs changing either which will cost you a couple of hundred, the problem is caused when the o-ring on a bung goes, you can buy a replacment bung from ford for a few quid