So there's a "known issue" in 14328 that Alt-Y does not give a "Yes" response to the UAC dialog. But for me the dialog does not respond to anything at all. Not the button, not the link, not the mouse, not the keyboard. So there are no useful changes I can make to any settings, and no programs will install. Any clues for how to get out of this? Where are the registry settings for UAC - I might be able to use a 3rd party editor that doesn't trigger UAC.
The build looks worth investigating, but I'll have to revert to previous if this carries on.