So after a month of having no maps for workouts and twice trying the recommendation to un-pair and re-pair my watch with no success, I finally "bit the bullet" today and went with the nuclear option, i.e. erase all settings and content on my iPhone and restore from an iCloud backup.
I wasn't using iCloud backup until recently as I was happily backing up to my Macbook which ensured that all music imported from my CD collection was backed up. I turned on iCloud backup a week ago in preparation for this attempt to get maps working again.
I have an iPhone XR running iOS 14.1 and a watch series 3 running watchOS 7.0.3. I forgot that I was supposed to un-pair my watch before doing this but it seems that this did not matter.
I just went into settings on my iPhone -> General -> Reset -> Erase all content and settings. After this had finished and the iPhone restarted, I went through the initial setup (connect to network, set up FaceID again etc) and was then able to restore a backup from iCloud. During the restore, I noticed that my watch had reset itself and seemed to be setting itself back up as it was displaying the progress ring.
The result is that it has fixed the problem on my iPhone - maps are now working again. The first thing I noticed is that workouts for the past week all now have maps whereas prior to the reset they did not. Workouts prior to October 16 still have no maps. I just went on a short walk to see if new workouts are recorded correctly and it worked - the map was there.
The Fitness app shows that workout data is still being restored from the cloud (more than an hour after the restore finished) so I don't yet know if activities prior to iOS14 being installed will get maps back or not.
As I suspected, all of the music that I have put onto my iPhone from my CD collection is completely gone. All music purchased from iTunes has been restored. I will try to selectively restore my CD music from a backup on my Macbook but doubt this will be possible. This is a real let down as I listen to my collection when I am driving.