Hi Owen,
I've published your example to Articulate TempShare. It would be really useful to know if this works for you - if you could possibly spare the time?
If it doesn't , it's my SL3 installation mucking up your example.
If it does work for you, it's probably something like my browser settings.
(These links will work until 22/06/2018)
HTML5 version
http://s3.amazonaws.com/tempshare-stage.storyline.articulate.com/sto_1cfpjaeq21de21sbb12jh9br11po9/story_html5.html
Flash version:
http://s3.amazonaws.com/tempshare-stage.storyline.articulate.com/sto_1cfpjaeq21de21sbb12jh9br11po9/story_flash.html
Cheers!