Twenty Ten: Problem with main menus

I have a WP website that someone has developed for me and I am trying to see if I can modify it to remove an annoving feature with the main menus.

It is based on Twenty Ten theme, but, has been modified to add a few extra bits.

The problem that when you hover over one of the main menu items it displays the drop-down items and expands the black background to a height of the drop-down. Ideally, I would like this not to happen as it obscures a large part of the page/screen. Probably not explained this very well, but, you can have a look on the site here….. http://www.prestburytennis.org

Any suggestions for how to sort this or where to look would be greatly appreciated