[smokeydan]

Thanks BScout - yes I had discovered I am getting more display options and apparently less CPU usage in PT if I connect the display over Thunderbolt rather than the HDMI - but it had not occurred to me that the 3 Thunderbolt buses might have different properties. I will add it to the list of configurations to try.
Cable lengths are currently 3m for the display, 1m for the OWC Drivedock and 5m for the Sonnet Chassis (although I have tried with a 1m cable for the Sonnet, it didn't appear to make any difference)