After downloading all the source files, searching for WT_ and finding no results it occurred to me this might be a browser issue.
Looking at the error it was coming from kiwitrees.js, and I think my browser had cached that file. Forcing a full refresh (SHIFT+F5) it has pulled through the latest JS file and the page is loading again.
Thank you both for your comments here.
My personal kiwitrees installation - http://familytree.derekjwhitten.id.au