@Graham_l the most economical way to use radiators is to switch them off when you don't need heat. With appropriate bedding it should not be necessary for a healthy person to heat the bedrooms at night. Different rule for ASHPs due them being most economical when heating at low output. Let the home temperature drop by more than a few degrees and the system will use an excessive amount of electricity to raise the temperature.
at more than 2 degrees, more energy is needed to heat up the rooms again than was saved overnight.
In addition, our bedrooms are always set to 16 degrees.
"at more than 2 degrees, more energy is needed to heat up the rooms again than was saved overnight."
Thermodynamically, that cannot be true. But in people's reasoning, apparently it can (as I often see this claim).