Do you think I need to do anything with terminals 7-8-9 on my boiler? At the moment there is nothing connected but I wondered if I need to loop any of these to make it fire up.
I've just spotted an error in my code. I was sending the SLAVE_CONFIG message instead of STATUS. I'll give it another try later and hopefully that might fix the issue.
I've confirmed it's working correctly now. Sending the STATUS message with ch_enable set to 1 is what it needed. The demand control into terminal 4 is working fine alongside the OpenTherm temperature control. Sorry for the false alarm on this.
I had to use terminal 7-8-9 in order to get my OTB to fire the boiler. And because that is undocumented, until I figured it out, I had to use the 240V trigger to fire the boiler and my VR33/OTB were simply handling the variable target flow temperature.