The simple way is 1 and 2 to 24 volt RT and L and N to L and N and E just safely terminated, in theory should be two cables one for extra low voltage and one for low voltage, in practice it depends how close the thermostat is to the boiler.
The on/off thermostat is only to stop boiler cycling in the summer so does not really matter what room as long as set a little higher to the TRV temperature, that boiler clearly designed to use a modulating thermostat if being used to control area temperature, I note the MODBUS and the BUS +/- so may be the thermostat can be connected to bus instead of using the extra low voltage option, but I personally do not know if they are using the same protocol, or where you would get the plug to fit back of thermostat.
If you wait some one may know if you can connect to the ebus, personally I would not take the chance without a lot of reading up first, but would use two cables so I could switch to ebus control in the future.
I did look up MODBUS but it seems to be more to do with a building management system using SCARTA and PLC's than OpenTherm so I don't think you will easy find the interface required. Seems to be more of an industrial system to a domestic, would expect to see it controlling fan assisted radiators etc in large office blocks where it does heating and cooling?
