OK I think I've got it licked. (PHD not yet installed and working so I'm not claiming victory just yet... )
After a fresh install of W8 (only took 30 mins or so from the recovery partition) and using the USB2 port the SSAG installs correctly. I'd imagine if I could get in to the bios and change the USB3 ports back to USB ports the result would be the same.
Some observations.
Using the USB2 port not only allowed the driver install, but the PID shows correctly as well.
It's no use swapping to a USB2 port once the install has failed, in my case at least I needed a fresh OS install (or maybe some serious digging in the registry etc to ferret out the remnants left over after driver and device removal.
Thanks for your support and suggestions guys, hopefully this will help someone else along the way.
