Not sure if anything changed in github but this morning I managed to get the plugin installed again. I restored a version from the plugins-old-versions folder which got Unraid showing the plugin with an update available and hitting update worked properly this time.
Thanks for the assistance, and if nothing else the whole experience highlighted to me the ongoing hacking attempt. I only opened up ssh access a few days ago to try and get some docker stuff going while I was at work. I'll keep that shut off from now on I think.