All sorted with many thanks to mikalou's reply in this thread.
http://forum.x-cart.com/showthread.php?t=40018&highlight=outputting+extra+ fields+on+cart
I was initially looking to retrieve all key fields and select the field I wanted in the template using smarty, but I changed the suggested SQL mod to func.cart.php to also include the extra_fields table, allowing me to retrieve the specific value by service name.