Viessmann Vitodens 100 30kw combi?

Help/advice greatly appreciated. The boiler is heating water on demand but not heating rads. I’ve tried resetting every thing even the external Siemens thermostat but this doesn’t help.
I’ve been in touch with Viessmann and they say it’s either the thermostat unit or the Flow Switch. I’m happy that the thermostat is working OK after resetting the connection in-line with the instruction manual.
Any ideas?