That's a topic for discussion Just wait for the "experts" to come out of the woodwork.I keep the heating set at 15C at night because I was told that it was cheaper than having it timed to come on and off. If that's not correct, I will set it to come on and off. I'm just watching my pennies.
Why don't you carry out an experiment? Run it for a week with the heating on at 15C overnight and then for a week with heating off overnight. Take meter readings at start and end of each week and compare the gas usage?
If your place does get very cold overnight, cavity insulation would be worth fitting - if possible. There are several free or low-cost schemes available.
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