Whether or not it helps I don't know but I have found the following through firebug:
<div class="box-4 deepest" style="min-height: 780px;">
When the minimum height is changed or deleted it fixes the problem. The figure of 780 changes depending on the articles so I assume it is taking a sum of the length of the articles.
Unfortunately I have no idea where to go from here. Have searched through files to find reference to this line of code with no joy. I am a complete novice at any sort of modifications like that.
Thanks
Stephen