Replace existing thermostat with diy solution
Why be married to their weird proprietary radio scheme? WiFi is available for the Pi.  

Put another Pi down on the furnace. For instance it can advertise (or better, not advertise) its own WiFi channel.

A thermostat is a simple switch.  Either it shorts two wires to call for heat, or it does not.  

Now in America we would just do all this with 24VAC control voltage.  But since Europe uses 240V thermostat voltage* you need to switch whatever the rating of the supply fuse/circuit breaker is, e.g. 16 amps.   A relay that size probably has a large coil, too heavy for the Pi to operate (especially when inductive kick is considered).  So have the "Furnace Pi" operate a small pilot relay.  The pilot relay pulls in the coil of a much larger relay. 



* so I guess that means Euro thermostats require Class I wiring to the 'stat? How inconvenient.  All the more reason to control the 'stat wirelesly.  Do the same with doorbells, while you're at it... 230V doorbells, *shudder*