Thanks, that link is helpful - even if it doesn't have all the information.
Sorry, I had not spotted the existence of the message length in your first post because I was focusing on how to distinguish one message from the next - which we haven't yet figured out.
Hopefully the stream of 100 bytes or so will throw up something useful.
The IBUS standard seems to use RTS and CTS for collision detection. I don't know if the Arduino serial interface can report the values of those lines. They might be a signal for the transition from one message to the next. Hopefully that won't be necessary.