Tracked it down. It is this javascript library call that is impacting the bulk uploader.
<script src="http://www.sisac.co.uk/forum/Themes/argentum_enhanced_smf20final/js/mootools13b_min.js" charset="ISO-8859-1" type="text/javascript"></script>
What is happening two different frameworks are being called which don't work well together mootools and prototype.
Solution would be if you can either that line from the theme.
Or just add a check to to see if action == bulk not load that library.