links back from image attachments

Hello,
Not sure whether this is a theme issue or me being dense, but…

If I have a post with a featured image, if I click on the image it brings up a bigger version of the image – so far, fine. However, the link below the title on that page ‘Back to …’ links back to this same image rather than the post I came from (the desired outcome, and as far as I can work out what the theme is supposed to do). I can see where the code is coming from in the theme (in the wpstart_entry_title), but can’t work out why the link is pointing to the image attachment url rather than the post url.

thanks for your help!