[Update]

Finally got round to investigating the wiring from the furnace in the basement to the old thermostat in the hallway. The three wires at the thermostat end measured up as neutral, live and switched return so it turned out I didn't need to use a no-neutral configured Shelly 1L as neutral was available. I used the 1L anyway and substituted the Shelly in the place of the thermostat and connected the thermostat to the Shelly switch input. The app or web interface now operates the furnace like the thermostat did.

The Shelly button is configured as "toggle". If I roll the thermostat dial so it switches on, the Shelly detects and switches on the furnace. If I use the Shelly app or web interface that overrides the switch and switches the furnace on or off regardless of the thermostat. Not sure if that's the best config as it means I can't turn off the furnace with the thermostat but can turn it on. 

Only real problem so far is that HomeBridge with the Shelly plugin is not detecting it for some reason. Have restarted Homebridge to no avail but not yet tried rebooting the Shelly device itself.