Drayton RF2 ignored by Baxi 105HE?

Hello, I have a Drayton RF2 digistat connected to a Baxi Combi 105 HE. I have had 5 years without a problem but for some reason now the bolier seems to not be "hearing" the receiver. The thermostat and receiver appear to be working fine with flame signal and LEDs all in working order however every couple of nights, the heating won't see the little green light of the receiver and won't switch on. I've been having to switch off at the wall to get the boiler to fire up. It will then work fine for a couple of days before it happens again...

Any suggestions much appreciated!