Second: But I kept getting an error that the folder needed to be empty. But it was brand new, and therefore empty. On my C: drive com, old files (2015+) could not be removed because they were, apparently, SYSTEM files. So I had to hack the registry to change permisions then delete the C:Reader directory. But that didn't work.
Third: So I searched through the many remaining Adobe (Photoshop etc) folders on C: for something else that might be interfering, without success.
Fourth: Eventually, I created the new folder (which I had expected to be created as part of the installation process. It would be a one line command in the installation tool) before installation. Then it worked.
I do not know which of these steps actually allowed the installation to proceed.
What a faff! Why cannot Adobe test their products properly before issuing them?