Overheating issue
FORD Fiesta Mk5 Hatchback (JH1, JD1, JH3, JD3)

Hi all,
I just changed my fiesta’s thermostat (and coolant in the process) as the car was not heating up correctly. I found that someone has previously removed the thermostat and there was no thermostat to replace. Nonetheless, I added the new thermostat. Nothing else was changed about the car.
After about 20 minutes of driving, the engine overheating warning light came on. I subsequently turned off the car before, letting it cool down before returning home.
Please could you advice why the engine may have started overheating and what action to take to fix the car ( I am keen to fix it myself)? Any help would be massively appreciated. Prior to adding the thermostat, I had no overheating issues.
(Note: The water pump was changed about 6 months ago. The temperature gauge appeared to move quite quickly, even decreasing in temperature by about 15 degree’s despite little change in driving speed – perhaps a sensor issue?. But why would that only become an issue after adding a thermostat. Also the internal car heater does not work, I plan on changing the heater control valve).