module height of left to match height of main

[Avalon]

Hi,

 

We are using the Edugeek Joomla 1.5 package and in general are getting on very well with it. What we are trying to tweak is change the height of the 'left' module so it matches the height of the 'main' content (which obviously varies page by page). I have set a background colour on the left module which is fine on the hompage but the background colour stops halfway down the page on other articles which is not great!

 

I have tried setting the height to 100% and auto but the issue still persists, does anyone know a way around this?