Uncaught TypeError: Only on Drupal edit pages
I had installed the dev version of jQuery Update 7.x-2.3+15-dev, which allows you to specify different versions of jQuery for normal and administrative pages: 



I'd done this because I had previously required jQuery 1.5 for something in the backend. It appears /edit/ pages are considered 'administrative' pages - (which seems odd to me)  and therefor were using 1.5 which broke the bigSlide.js menu. 
Changing the admin pages back to 1.7 fixed the issue.