Problem with central heating, could it be a fauly room thermostat, urgent!!?

Hi, this week my ideal isar combi boiler has developed a problem. The problem is that it now only supplies the hot water but refuses to supply heat to the central heating. The boiler displays "C" on its screen with a white light flashing constantly. When i switch the boiler on and rotate the room thermostat from 0 degrees to 27 degrees sometimes the central heating seems to be fired up but this only occurs for a few seconds and then the light continues to flash constantly. Any suggestions would be great. thanks

kenny