Vokera Linea 28 boiler fired up when heating "off"?

A Vokera Linea 28 boiler was set to "domestic hot water" only, ie the heating was set to "off", but the boiler fired up, heating the radiators (as if the frost stat had cut in). The code showing was "C0" with the indicator green, not flashing red.

Turning the control to "OFF/ RESET" shut the boiler down and later it seemed to be OK on domestic hot water only.

Anyone know what's going on?