So.. Ian got my M5 cable and although I'm still awaiting the results of rigorous testing, early indications are that it's fine!
I've tried using my spare master comms cable into the comms output but that seems to make no difference. I'm not completely sure that it is a comms cable, but it was in my bag of Meridian cables, so maybe

I'll try again when I get my cable back from Ian.
With some messing with the legacy config program and the DSP33 speaker settings, I've managed to get them to play music when connected to the 568, but the 568 won't make them change volume. I've set 2 of the speakers to "volume control" main" which I'm guessing is right? or does the slave speaker get set to secondary?
Also, I suspect they're on an older firmware, where do I get the latest firmware for the 33's?
Once I have my cable back, I will report further.
David