Just a quick update:
My computer is on 24/7 and this morning the drive had gone to sleep (even though all the power management settings tell it not to) so the only way I could get it to show up was unplug/replug as usual 
I’ve now plugged it into a USB 2 port to see what happens.
SO the HPET settings seem to at least allow it to be recognised in a USB3 port - but that’s no good if you can’t see it after a couple of hours.
