I deleted the download and re-downloaded, to no avail.
I tried custom installing again but this time ticking the box for VST2 (which I didn't want) and interestingly that took me to a screen showing proposed directories, which wasn't the case when only ticking VST3. Shouldn't that screen also appear when only ticking VST3? There were a lot less errors, but still a couple and these were sufficient to bugger up the install again. And this time only one icon appeared on my desktop - for a 32 bit install!! At least before there were two, one for 32 bit and one for 64 bit, which is my standard. What the f** is going on??
I've raised a ticket to Support.
This is very, very, annoying as I never have trouble installing new software; and Ozone 5 and 6 went on sweet as a nut as I recall, and I've not changed anything on my PC - I've even held back on going from W7 to W10...