Some ideas Alloytec:
Worst case - Could be your thermostat housing gasket/O rings failed. The lost radiator fluid may or may not show at the bottom of the gearbox. Mine didn't.
Radiator cap is leaking vacuum and not sucking back from overflow tank. Splits cracks perished distorted seals - three surfaces in the cap.
Crust built up or damage on two surfaces of the radiator filler opening stopping effective seal for vacuum and water return.
Hose from filler pipe to overflow tank is split, cracked, perished and losing water when venting, and thus losing vacuum when returning coolant to engine when engine block cools down. Over flow tank level is the zeros and circles near the bottom of the dipstick, about 550ml.
Hose in overflow is not far enough into the overflow tank, should be immersed - no return fluid vacuum present.
Check the radiator is bled free of air (bleeder screw near battery end of radiator top tank) and the heater tap is working, not leaking, and no air lock is present.
Hoses to and from radiator can leak when hot and be OK when stationary.
Various O rings, 2 small (heater) and one large diameter [Engine coolant cold feed]) at thermostat housing, and as Philmecrakum has suggested the two O-rings at the filler pipe.
Water pump gasket failed.
Water pump leaking at shaft seal.
