Boss therm br1 not communicating properly?

Hi, we have a boss therm br1 which was fine until batteries died. We replaced and all was ok then they decided not to communicate. We found answer on here (thanks for that!) and did the learn and test scenario and all came back on and lights ok. Now though the lights are all on but the heating comes on intermittently? Thermostat reads say 20 and the triangle on but heating not coming on, then magically starts up and then won't go off if we turn the thermostat down! it doesn't come on with the timer either properly? We have tried yet more new batteries but to no avail. Boiler works perfectly ok and the timer works ok as well as hot water fine. Any suggestions? Or is it a new wireless therm needed? Many thanks for any help as getting colder :-)