Traditionally builders always seem to put the main thermostat in the entrance hall, but that really isn�t the best pace for it, as that is often the coldest place in the house, often with a draft from the front door.
If your radiators all have individual thermostats, then you can adjust each room to have its own comfort level, but if your rads are set fully on all the time, you may want to reposition your thermostat in the room where you spend most time in the evenings. Bedrooms don�t need as much heat, and bath rooms and shower rooms need a little more.
If you can set an acceptable level of heat in your sitting room, chances are the other rooms will also be comfortable if the system was designed properly.