in very cold weather conditions is to set your required day and night temperatures, and let the room thermostat maintain them - leave the boiler control set to 'constant'.
That way the boiler will cycle on and off as the room-stat calls for heat. The temeprature will be maintained,and your house will be warm.
If you switch the boiler off at night the house will cool down, and in the morning the boiler will have to run full tilt to get the ambient temperature back up again. Likewise, if you switch the boiler off during the day when you're at work, it will have to do the job all over again late in the afternoon to have the house warm for your return. You're heating the house up, allowing it to cool, and heating up again - it's an inefficient way of doing the job.
This needn't apply when the weather is milder - I'm talking about cold snaps.
During the recent very cold weather my boiler was switched to constant, and at night the room stat was turned down to below 15 degrees. The boiler cycled gently on and off during the night, and didn't have to work so hard in the morning.