Boiler has stopped heating

Wireless thermostat, ext kit and numerous trv’s have all been working fine. However last 2 days when Tado calls the boiler for heat, the app tells me it’s heating (3 dashes) but no heat and boiler isn’t fired. Same when I try manually.

Weirdly all ok on hot water. Schedule runs fine and if I call manually for hot water boiler responds.

Does anyone have any idea?