Thanks guys.
I have just written some crude code and now the master can do the scan of the bus and each slave reports back its address, its serial number and some information. At this stage is all very loose, but the information is 20 characters of information, such as "Opto Input - Rev 01".
I just started with that too see how it would be transfered over I2C etc.
In my slaves I have onReceive and onRequest interrupts, and all that appears to work fine.
So the Master has the following modes:
The ability to auto assign addresses as the modules are plugged in 1 by 1.
The ability to clear addresses from all modules and assign them to 119.
The ability to specify a specific serial number and assign it to a specific address.
The ability to interigate the bus, and receive back each modules information (Address, Serial and Information).
All these appear to work correctly.
I have only implemented this on 1 module so far, so will copy and paste in the bits to the rest now.