Add a "Recently updated products" page

Hi

I am trying to create a page showing all my recently updated products (I have 10 000+ products and my customers want to see when I add new stock to items). I used the new products page as a template and followed the instruction in the wiki on how to create a new page. This has all been successful.

I now have to figure out how to change the sort order for the new products to using p.products_last_modified. As far as I can tell if want to do this I need to create a new sort order in the drop down list, but I am unsure how. I think I am suppose to alter these files, but unsure how:

/includes/modules/pages/updated_products/header_php.php (my page)
/includes/modules/listing_display_order.php
(and maybe more)

Any help would be appreciated.

Zen cart 1.3.9h (upgraded from 1.3.8, recently)