21 Tilly.
Fit a dummy roomstat for him to set to 18, then put the real one somewhere he won't notice (easy job........ only 3 wires ...... sometimes 2)
It kind of doesn't matter what ours is on BECAUSE IT DOESN'T BLINKING WORK PROPERLY! If I have to tell Mr BM to get a plumber out again, I might be taking a torque wrench to his valves!
I haven't got a thermostat...but if I did,I'd most likely have it set to 20. It's 18.5 in here at the moment and I could do with a bit more warmth. When I was young,my dad always set the heating to about 25-27......we all roasted!
The thermostat is at 24, but the radiators that are turned on are only on 3 (out of 5) and the wood burner is lit too, probably not the most efficient way to do it though.
It's a lot to do with how much you move about and what the ambient temperature is; if you're moving you create your own heat, if the air is being warmed up you're skin is aware of getting warmer regardless of what the temperature actually is. That's why people who are more sedentary tend to keep upping the setting to make the heating switch on.
With decent insulation, Tilly, you shouldn't have to leave it on overnight. Goes off at bedtime, and on again an hour before you get up maybe.
Barmaid .......... you do need a heating guy.......... it may be an expensive circuit board, but it could easily be a cheap thermistor (thermostat kinda thing)