Towerstat Receiver and Transmitter both look fine but boiler not firing up?

Hi

I have a Towerstat RF fitted on a Heatline Boiler. Boiler appears fine, firing up for hot water etc when required. However, although thermostat receiver and transmitter appear fine (everything lit up etc), it doesn't actually fire up the Central Heating. Any ideas? I have tried resetting the system but still not working? Has worked fine for a couple of years then stopped - changed batteries in transmitter and all reset but still not appearing to talk to each other.