Re: Mechanical solution
"...down to 15 degs when we all go out and back to 20 when back in."
If you can 'Yo-Yo' your house's temperature down and up by 5°C in the time it takes to go out for burgers (an evening out), then that's an indictment of its energy leakage.
We could pull the main breaker (zero heat), go out for the evening, and come back several hours later to a house perhaps 2°C cooler. If we went out during a crisp (cold) sunny winter day, cutting the power, we might return to find it had risen to +22°C or +23°C from the passive solar gain. Our thermostats are programmed to reduce the nighttime temperature to +18°C, but it takes several hours to get that low (just a 2°C drop).
Being *able* to Yo-Yo the temperature up and down quickly isn't a good thing. Ideally your house would have a much longer thermal time constant.