Ahah! Four hours trying to get 'round this and 5 minutes after I ask for help I get it working!

 

I installed DevToolBar into Internet explorer and turned off the cache (even though my browser was set to look for a new version of the page on every visit) and it started working.

 

Just FYI, this affected both IE and Firefox and ctrl-clicking refresh / Shift-F5-ing didn't force a refresh whilst in the editor.

 

Hope this helps someone else.

 

Craig