My diahatsu charade detomaso overheating


Hi... I took the housing off the engine block for the thermostat which had the top radiator pipe attached. what i found was no thermostat. just empty housing which is in fact too small to house any thermostat that my local store supplies. The car shows signs of in fact having a thermostat. e.g. the heater works perfectly and the top radiator pipe hardens after the car heats up, which indicates the thermostat is closing.
I am just an amatuer but even so this makes no sense to me, please, please help
My car is a diahatsu charde detomaso, 1600cc, 1994.

Thanks ben