Unfortunately Library Check does not show any problems.
BTW I should have mentioned that the other .mobi books show up fine (I know my attached picture shows only .epub books but that's just because I sorted them in that order).
I've also looked at the individual metadata in some cases. I notice that I've got an 'add format' button somewhere in there (top right iirc) but I've not used this before and don't really understand where it takes me...
Just wondering if I should just add 3 books to a new library - i.e a known good .epub, a known good .mobi and known 'bad' .mobi and try to figure out what's happening.
Thanks for trying to help and any other comments you might have would be appreciated. Regards. Allan