Well, up to a week ago I was having the fall back to 14°c
One morning in particular, the batteries went flat in the thermostat, and when I got up in the kitchen it was cold.
Decided to try leaving it on 18°c all the time, yes, that means in the night it may come on, but my train of thought, is perhaps it's more efficient keeping the slab warm rather than doing a bit heat-up every morning.
As a coincidence I've just had the heating bill/usage through and I'm nearly a third lower on gas usage.
Previously I'd been setting the thermostats to around 19°c but being on 24/7 it seems permanently warm.
Might be a fluke, but I will run this way for another month to see.