Baxi Solo not responding to room thermostsat?

My Baxi Solo 2 50 RS does not switch off, even when the heat in the room greatly exceeds the temp on the room stat. The stat "clicks" when the dial is turned, but the boiler stays lit. The only way I can stop the house becoming a sauna is to set the dial on the boiler extremely low, and have the timer switch off after a few hours. can you suggest what is wrong and how I can test it.