Thanks for the reply.
I have already tried Firebug which shows no errors. I have also tested the event handler is being called by adding an alert. All works fine. The script takes no data and either works or doesn't. The page data on which it operates is unchanged.
I will try removing the https component of the issue as Gustaf suggested, and will also replace the script with one that doesn't call any YUI3 components. (I have the yui-min.js included as [[js:yui-min.js]] in the xowiki::Form and am relying on it to autoload the component files for me from Yahoo. I did this because referring to the yui-min.js file from the Yahoo url in the formpage didn't work.)
When I previously had problems sourcing YUI components I saw errors in the IE console and Firebug, but this time I get nothing.