I've just done an experiment as I have the place to myself. Normally the thermostat is set to 21c during the day and then down to 18 at night. The place is pretty well insulated and so the heating rarely comes on before I get up around 7ish, by which time the temperature is usually around 19. So the heating hasn't kicked in but there is a bit of a chill so I finish up forcing the heating on. As I've got the place to myself for a few days I put the thermostat at 21 for all periods including night. No changing it, no putting it up a bit. The result cost wise has been less than 10p over a 24 hour period. The benefit is the place is a comfortable temperature all round. The boiler doesn't have to work to get from 19 to 21.