Looking at the BCM signal today it is def serial RS485, and looking at the BCM board I have found the two transceiver chips are Linear Tech 1487.
Datasheet below.
www.solarvan.co.uk/insight/1487fs.pdf
Now just got to capture and analyse it.
I'll order one of these chips so I can drive it with a picaxe and send some data on the rs485 bus.
Another useful device I've just ordered to play around with is.
http://www.robot-electronics.co.uk/h...rs485_tech.htm