1. Remove the radiator cap when engine is cold. Rev the engine. If the coolant fails to 'rise' up the filler neck you can assume the water pump is faulty.
2. Failing that drain and flush the engine block and also reverse flush the radiator.
3. When the thermostat was fitted was the toggle [the hole in the thermostat back-plate] fitted uppermost? If fitted downside it will fail to open - air remains trapped inside the engine block and cannot escape since the hole is too low within the engine block. Hot air will not open the thermostat.
4. If the replacement thermostat is not of correct type for a daihatsu [long reach] it will fail to open. The coolant will boil!