Cannot customise my site - blank screen

My site at http://www.studentconnect.co.uk, using the twenty-fourteen theme functions perfectly - however when I came to update some elements and tried to customise my site all I got was a blank screen. I saw there was an outstanding update of Wordpress, and installed that, but still I only get a blank screen when I try to customise. I have enabled support mode so that you can access the site.

Thanks for looking!

Regards, Paul