We're stuck with keeping the house at temp all the time, i did experiment with a small 2C setback and it takes 1.5 days to get the house back up to temp.
Using a very small (cheap and nasty) ASHP which can just about manage down to freezing.
We've been very lucky so far this winter and only experienced -2C for a few hours at most.
Must do a white up sometime of how much hassle it's been to get working efficiently.