Beware of this upgrade because, since installing, I have lost all drop down menus from the administrative menu at the top of the page. 

Not so much of a problem for accessing the Admin and Host menus because the links work and take you to the admin page and provide you with an alternative location.

However, if you want to use the 'Modules', 'Pages' and 'Tools' menus these have no link to an alternative location, so if you lose the drop down menu you lose all ability to work on your site.

I don't know if this is a conflict with my skin but I think there is a real usability issue in having no alternative means of accessing the three menu items I refer to.

Please can anyone advise how to fix this?

now that is a catchy post title.

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Discover Venice

Then you have nightmare!

I had cleared the cach completley..