Is the thermostat just in one room? I've been tweaking our heating for a couple of years and have it nicely balanced. So what the thermostat is set to isn't necessarily what the temperature of the rest of the house is.
Our thermostat is in the coldest room and is set to come on when that's below 20 during the day, I cant remember what the night time setting is but it's lower. Then each radiator is thermostatically set so it only comes on when the room it's in drops below a specific temperature.
If our thermostat was set to 17C in the daytime, our house would be too cold.