OK, panic over!!!
After a whole day of trying various different categories in both the root and default categories, I decided that something else must be afoot.
So, I checked in the database and found that in table “catalog_category_entity_varchar” there were two entries for the root category (Root Catalog and root-catalog)
For the default category there was only one entry.
So I deleted the Default Category and recreated, and hey-presto two entries appeared - Default Category and default-category
Now everything works fine. Not sure how that happened but…
So, after more testing I think I can safely say that in order to get the navigation menu to appear you need to put your categories in the Default Category and NOT in the Root Category.
Hope this helps