OK, the more I look into it the more confused I am getting.
I've updated to the latest version of Wordpress. I've checked all the other listings and they seem to be fine. They display in the correct category when you access the slug address.
I've created a new category and added this post to that category. Again when you look at the slug address the post does not appear there. I've tried removing the images from the post in case there is an issue with those but it still wont appear.
It does appear in the parent category though, even though it has been removed from there.
Its all a bit confusing! :-(