Solved eventually:
Wait about 2 hours and reboot your mac, then open the app store and an Xcode update will be available to you.
(This bit about waiting an arbitrary interval was not intuitive.)
conradb
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution
Last seen
User for