If you go to www.sdeyservices.co.uk/test you'll see that I'm trying to extend the height of two columns.
At the moment the problem is that the columns stretch so far that they extend into the footer. How can I set the height of these elements so that they only extend as far as the top of the footer?
Columns are a very common issue for CSS design. There are a number of questions/answers already here that you can look to for techniques on how to do this. A lot of the ones I've answer have to do with 3-column layouts, but this question will probably be most helpful for your situation.