SOLVED. This took a while. I didn't find any references to 3rd party security products in the device manager hidden items or in the properties for the network adapter. I disabled power saving and restarted it without success. A new adapter had the same problem. Eventually I just tried SFC /scannow from an admin command prompt. That fixed the connection.
Then I found the likely cause, the system had media view, conduit search, better surfing, and a bunch of other malware, so once the system got online they started to pop up all over the browser. Windows defender couldn't run, nor could on screen keyboard or ccleaner, and a load of options and files were access denied to the admin account, even after taking ownership.
Taking off the...