Individual Category Sort Ordering

Hi there.

I'm trying to figure out how to create an individual sort_order for 1 category within my shop. I'm looking to make this category sort_order by date_added.

I've tried to find a solution in the forums and in Google but have come up stuck. My version is 1.5.6.1 and I've tried to edit the following:-

catalog/model/product.php
$sql .= " ORDER BY p.sort_order";

I only want category_id '11' to be initial sorted by date_added with the newest results at the top?

Any help would be greatly appreciated!

Kind regards,
Chris.