my standby light keeps coming on?

i have an FER Falcon 0047L60046 boiler in the really cold weather the rads only get luke warm then the standby light comes on. This repeats this the whole time the heating is on. I have no thermostat fitted in the house and the one on the boiler itself turned right up. the water from the taps comes out hot and normally the heating works fine but only in thie cold spell of weather has it started really playing up. Can anyone help or is it new boiler time