Yup, tried it all.
However, in the hope that it helps others, I found the solution after browsing the PDApp.log in Program Files\Common Files\Adobe\OOBE\PDApp after finding that via Sysinternals' ProcMon.
The error in the logs was
02/18/15 13:11:50:551 | [FATAL] | | ASU | PIM | PIM | | | 5332 | Failed to create backup folder C:\Program Files\Common Files\Adobe\OOBE\PDApp\core
02/18/15 13:11:50:691 | [FATAL] | | ASU | PIM | PIM | | | 5332 | package installation failed and the return code was 50
Doing some more googling for that I found a thread elsewhere that suggested it could be solved by 1) using the Creative Cloud Remover toolhttps://helpx.adobe.com/creative-suite/kb/cs5-cleaner-tool-installation-problems.html and then 2) restarting in Safe Mode with Networking; 3) loging in using the Administrator account, and then 4) re-running the install. So that worked, and it might well be that not all the steps are necessary, e.g., if one's account is already an 'admin' account.
The person who came up with this solution says that he has to do it this way every time there's an update. What a joke.