Thanks Fabio
Solution was to go into the Windows Programs/Features, find Cubase 8.5 and do a REPAIR. This clearly showed that it was looking for the original installer that had been extracted to a temporary location a while back, but had since then been removed by a registry/system cleaner, probably CCleaner.
I simply pointed the Repair to the location of the installer that I keep on my Studio partition and it completed. I then was able to apply the update to 8.5.20 successfully.
Only issue still not fixed (since introduced in 8.5.10) is “Video service not responsive” on startup. 