[support] Why would an upgrade to the Plus_1 module mess up some (javascript?) pull-down menus?

I have been slowly working my way to finding which module update had caused this problem, finally found it today, updating the Plus_1 module from version 6.x-2.6 to version 6.x-2.7 breaks the pull down menus on this site. The working site is:- http://www.globaloptimists.org/ I have a copy, with Plus_1 updated to 6.x-2.7, at:- http://globaloptimists.isbd.net/ As you can see the menus at the top are permanently pulled down on the updated site. Can anyone suggest where I should be looking to see what is causing the problem? It's not urgent as I'm quite happy to leave Plus_1 at version 6.x-2.6 for the present. -- Chris Green