Adding a sort option is described in this document:
https://devs.x-cart.com/basics/itemslist_introduction_--_showing_products_on_a_page/adding_new_sort_by_opt...itemslist.html
The example uses a date, but if you can identify the SKU field, you should be able to use that. Unfortunately, this is where I came unstuck, but as Steve said in his last post, the option I wanted was already there. I just didn't know it at the time.
Welly