Seems odd that the boiler is constantly running. There are two demands, one is the room stat, the other is the stat in the boiler which measures the heat of the water circulating in the system.
Our place leaks heat massively (Victorian town house); if it's below zero outside, the max inside temp we can achieve is about 17 degrees. So the demand for heat from the room stat is constant (although it's set at 18 degrees, 21 is a sauna!)
However the boiler isn't working constantly as it can maintain 70 degrees water temp without having to be on all the time. I guess if we had more radiators, the house would be a bit warmer and the boiler would be working harder.