[Eddi] made a great find! This is a discussion on EEVBlog on decoding the interface on an LG system. While I think the comms protocol is different, there is a suggested schematic for a Dual Comparator Serial Bus transceiver that looks like it is exactly what we need. I have ordered some LM393s - I'll build the circuit one they arrive. https://www.eevblog.com/forum/projects/where-to-start-trying-to-decode-a-data-stream/25/
In the mean time, I'll pull the second unit I have out and see if I can get it running in slave mode. That was I can listen to it's TX pin, and see if I can work out what it says.