Drayton RFI Digistat?

My Drayton Digistat SCR has a constant red light and does not fire up the boiler through the wireless thermostat when the fire symbol comes on.
Have changed the battery and tried to relink signal but still the red light is on constant.
I can ony fire the boiler manually by pressing the '2' button and the green light comes on, but have to then manually turn it off once temperature is at the right level.
Not sure what is wrong can someone help!