Hi Simon,
I’m not sure about that cause of the problem, but I can give you some info. When the mappress shortcode is processed the plugin writes out some fields (like the directions start/end) and its javascript and css. In that page I see the fields are there but not the css and javascript. To narrow down the problem, you might try just putting the map on a plain page instead of in the ‘popup’ window and see if it displays properly.