The white div class="contentbox" is as high as it can be. The black band underneath is where the div with the links Contribute, Press Releases and Advertise are.
The problem is that you cannot see these links as the stylesheet code for a:link, a:visited and a:hover are all color: #000000; in other words black on a black background.
Either change these colors or change the color of the div. Near the bottom of your html file code is:-
<div align="right"><a href="http://www.ukslc.org/Contribute.html">Contribute</a> - <a href="http://www.ukslc.org/Press_Releases.html">Press Releases</a> - <a
href="http://www.ukslc.org/Advertise_On_Ukslc.org.html">Advertise</a></div>
Change the first line to say <div align="right" style="background-color: #FFFFFF">...etc. ie white or another color.
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PS I notice you have added class="footer" and added .footer a:link, .footer a:visited color: #25668D; } to the stylesheet.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.