I've got distracted a little bit with my Mechanical Tulip. No worries. I am working on this one right now. In the meantime, I've used my OpenTherm experience and knowledge to create an OpenTherm shield for Arduino UNO. It's an exact copy of OpenTherm component on this board. But can be used right away with any Arduino UNO board.

It will allow you to control your boiler without a thermostat or act as a gateway between boiler and thermostat to monitor their activity! Now testing it. Will be soon available on my Tindie store.