Time for another update.
Been working on the software this weekend. I have the 2 way video/audio chat working as a UWP application (hacking the Microsoft simpleCommunication sample), and have speech synthesis when you press the bell it says 'calling' and after 30 seconds if no answer from inside it tells the caller there is no answer and goes back to listening mode. Have hacked in all the GPIO control for the LED, LDR, amplifier control and general IO for switches, and have MQTT messaging back to the server working.
Finishing touches will be to integrate the universal app with a browser for access to the main automation system (HTML5 client) which will use websockets to communicate between the browser and the chat app (and MQTT between the client and server). And also to work out the physical mounting into the letterbox so that it looks half decent. And will also add SMS notification when the postman posts a letter (also handy for couriers - video chat over the internet is a possibility).
Have found the perfect mini PC to work as the indoor station (although it can work from any Windows device):