After much appreciated help from David and Beetroot, exploring various avenues, it turned out to be a conflict with a jquery-ui 'tabs' script. The developer told me to remove this:
jQuery(document).ready(function(){
jQuery(function() {
jQuery(".tabs").tabs(".panes > div");
});
});
from custom.js
Guess plugins need to come up with more unique names for their elements!
Thanks everyone.
