Pump not working if Central Heat only, called for?

The central heating would not start, it would appear that the boiler fired but the pump did not function. If I over ride the hot water the pump works and the central heating starts to warm the radiators and will continue even if the hot water is then stopped. I then replaced the Danfoss Lifestyle LP522 and everything appeared to be working correctly.

Now 1 year on the problem has re-occurred. The room stat is a Boss Therm BRTRF which communicated with a BR1 via wireless.

What could the problem be?

Allan
October 2018