The first time I just installed it over the previous 12.3.4.204 installation, and the installer hung.
I then tried reinstalling 12.3.4.204, and it told me there was a more recent version installed!
As the usual Adobe test page no longer works, it just redirects to the EOL notice, I tried a few other Shockwave test sites, none of which worked as they should, so I assumed the install was corrupt.
Then I uninstalled it, and tried reinstalling 12.3.4.204 which hung again exactly as the 12.3.5.205 installer had done!
I wondered if it was trying to connect to something online and not succeeding, so I uninstalled again and tried installing the latest version again with the internet connection disconnected.
This time it worked, and very quickly!
Whether my theory was right or not I don't know, but it seems to be installed OK now.