window.addEvent('domready', function() {

     $('london_link').addEvent('click', function(){
         $('windsor_office').className = 'hidden';
         $('london_office').className = '';
     });

     $('windsor_link').addEvent('click', function(){
         $('london_office').className = 'hidden';
         $('windsor_office').className = '';
     });

});