Yale alarm? Can you link?

Vailant boiler - I wonder if there's a debug interface you can use. Not knowing specifics of your model, i foudn this: https://github.com/jayme-github/esphome_vaillant

Codex can help probably further heh.

It's a Yale Sync

https://yalehome.co.uk/yale-sync-hub

It's not really something I want 'home rolled', I just want it to work.

Same goes for the boiler really. If it stops working I can phone Valliant and they'll send someone round to fix it.