I decided to trash that page and redo it. Everything appeared fine on the draft (although interestingly Easy WP SEO wasn't pinking things up). When I published and tried to view the page it said it couldn't find the page.
What I have noticed is that it is trying to find the WRONG URL. It is trying to find exercise-for-osteoarthritis slug rather than exercises-for-osteoarthritis.
I have checked the slug details for the page and it is all good both under edit and quick edit.
I did have a page exercise for osteoarthritis but that has been trashed.
Please can anybody help? Thanks.