Thanks. I’m a WP novice and struggling to work out what’s required, so sorry if I’m missing something obvious.
Are you saying that the “in_same_cat” parameter of previous_post_link isn’t relevant in this case? I can change the link text and it shows up, so its obviously doing something (and not being overriden by the function).
The twenty_eleven_content_nav() function is here https://pastebin.com/0wHuuTbC. As far as I can tell there’s no ability to keep navigation within category.