It is something to do with the way that your IIS server is responding to our cards crawler’s user-agent (Twitterbot).
If I try curl -A Twitterbot http://www.wales.nhs.uk/ogdata/media.html the resulting page is a standard 404 / not found HTML page. Omitting the user-agent returns the page with the markup as expected. It might be an issue with request filtering in IIS.