Mmmmm, interesting.
I'v been digging and I think I want something like CAN-BUS.
The collision problem is addressed with message priority, which in my case would increment with resends, and devices are not addressed but listen for pertinent data as defined in the packet header.
Essentially everything is clear to send when the bus has been idle for some time.
Any device with data to send will do so simultaneously, whilst monitoring its own progress.
1n's always superseded 0'z and the first byte of the message is always the message priority.
If a talking device sees a 1 on the bus when it intended to send a 0 then it will assume that it has been talked over bay a higher priority message and stops sending, increment its message priority and weight for the next slot.
This bit checking continues for the entire packet which will prevent massages with the same priority but differing data form conflicting.
If anyone is aware of a software implementation of this kind of protocol for Arduino I would be most grateful for a pointer.
Failing that I think I will probably start with SoiftSerial, for the interrupt structure and work from there.
I suspect this will be harder than I am currently giving it credit for.
Al