An update on my previous post...
Although my installation worked, I was disappointed with the performance. I expected more boiler run time at lower output as the system modulated, but instead it seemed to switch the boiler on and off more frequently than the conventional relay-driven control. I tried to find a fault but couldn't. Room temperature control was OK, but in the end I reverted to relay control as I was concerned the number of boiler starts might cause premature failure and I didn't have time to mess about with it. I meant to come back to it, but we then moved away.
As mentioned before, I built an Opentherm gateway -
http://otgw.tclcode.com/ - and found it quite useful in monitoring the messages back and forth across the Opentherm network. My next step was going to be to tweak the voltage level at which the gateway triggered to see if the signal from the bridge was hovering around the trigger level and causing false starts.
My current boiler does not support Opentherm. When the time comes to replace it I plan to have another go, but may go for a more OT-friendly brand.