[schapplm]

I have not found details of which file/s the watched indicators are stored in so have not found my easy copy, run wizard and re-install file/s option.

The watched flag is stored inside the Kodi video database, which is e.g. ~/.kodi/userdata/Database/MyVideos90.db. So maybe restoring the database file already works for you.

In my search I came across WatchedList, which I have installed and it runs automatically 5 minutes after I start up Kodi 15.2.
I see WatchList start and update messages displayed at the top of my screen, but for all items the reported number of saves is 0?
What am I missing?

Is only the number of videos newly marked as watched zero or also the total number of videos in the WatchedList?
Both values should be displayed if you enable the debugging mode (see here for settings).

Am I going down a false road, my thoughts were that I would create an external to Kodi copy of my watched TV/Movie watched indicators using WatchedList, run the Build Wizard, install/then use WatchList to re-install my TV/Movie watched indicators but cannot see an option to import the copy created by WatchList??

Your described workflow is correct.
The addon saves the watched indicators in a database file, usuallly ~/.kodi/userdata/addon_data/service.watchedlist/watchedlist.db.
You have to make sure that this file still exists after the re-installation of Android/Kodi (I don't have that system myself so I don't know the steps and am not sure what the build-wizard is).
Alternatively, you can set the WatchedList database to be on a network share or on a remote mySQL server. Then the data is not altered by a re-installation.