Can't get a link to downloadable image to display properly

Hi,
We have a calendar of events which is both displayed as an image on the site and also a link to a downloable version as you can see on this page:

https://www.midstaffsmind.co.uk/services-offered-at-mid-staffs-mind/526-2/

The problem is, in order to make the clickable link to the PNG file work, I had to copy and past the code from a previously working link in the ‘Text’ tab of the post editing window and tweak it with the new link.

When I try the usual method of clicking ‘add media’, uploading the PNG file and selecting ‘Link to media file’ in the attachment display settings it never seems to work.

I have a workaround at the moment but am I missing something obvious? Maybe I’ve forgotten the usual way to create a text link to a downloadable file?

I’ve just updated to WP 4.0 but this has been going on a for a while.