Then it is empty. It should display the contacts that you added in the main window. What happens if you double click the list folder and open the edit dialog box? Is that blank too?

When I double click the list folder, the dialogue box asks me to enter an email address that I wish to add to the list. When I add these details they appear in my personal address book but as you suggest the new list in fact remains empty. So the question becomes "how do I create a new list of email addresses?" I have just part answered my question. By dragging an email address from my personal address book to my new list. It seems I must add a new contact to the personal address book THEN drag it to the new list where it appears as a second entry.

Contacts must be in the address book that the list is created in. It is not a second entry. It is a contact that has been added to a mailing list.

This explains mailing lists. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Create_Mailing_List

If you are wanting different address books to separate contacts then create a new address book.