Why might a central heating programmer no longer override a room thermostat?

My central heating and hot water is governed by a Danfoss FP715 Si. Using this I can set the time each day when the central heating and the hot water come on and off. When the central heating is on it is further governed by a Honeywell DT92E wireless room thermostat in the living room. This was working fine until a few months ago. Now the central heating only listens to the room thermostat. It does not matter what the Danfoss programmer is set to, if the temperature in the living room falls bellow the value set on the thermostat the heating clicks on, even at 3 in the morning.

What’s gone wrong? How do I diagnose the problem? How can I fix it so that the heating only comes on when the Danfoss says it can?