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Upon further testing , I was able to set both ETTL and HSS.
I do recall setting ETTL on prior occasions but my omission was not to save the settings before taking the first shot. If you do not save, then the slave will revert back to Manual. I haven't downloaded the images to confirm darkening on one edge when in HSS, but it does fire in that mode ok. Setting the 600 EX FEC to 0 allowed FEC control from the camera , and tests with observation of the histogram and image highlight indicator (for overexposure) confirmed correct operation.
One thing with HSS; if the Master and Slave are set in HSS , and a change in camera settings results in a shutter speed which is within normal sync speed, the HSS will drop out accordingly, and will need to be manually activated again via the saved settings if required. Nothing major , but something to keep in mind.