Manual operation is UP & RUNNING.
After wiring the relay and tidy up a remote unit is ready. By publishing ON to boiler/onoff the boiler is turned on.
I have changed my mind. All the ESP8266 are going to be names Remote Units. All the remote units will report temperature and support an output connection.
The same code will run in all the Remote Units. The relay will be installed in the unit next to the thermostat, the other units could have a LED, to signal the boiler is ON, or nothing at all.