Managed to get it to install after deleting the various folders in Cache and elsewhere mentioned in this thread. This is typical of Apple really. They make nice hardware, but when it comes to their software it's pretty flakey, often not very intuitive, and while perhaps some people push the line that Apple software "just works" that's certainly not my experience. The frequent changes to App Store Connect documentation, web interface, certificate and signing rules, etc etc., demonstrate a blatant lack of respect for the Apple developer community. Having to clear 50GB just to update a package that it already installed is, simply, poor design and poor implementation, but more fundamentally it just shows the Apple philosophy: "You want to develop for our platform? You need to work or beg for the privilege"