Thermostat Control?

I have a Digistat RF1 wireless unit which was replaced in Feb. since replacing it, a red light appears on the unit by the boiler. I have hot water and heating but the unit is no longer controlling the temperature of the central heating. I have to do this manually by controlling the heat of the radiator via the boiler itself. I have tried re-commissioning the unit several times but the red warning light remains. Any ideas