Graeme,
It sounds like you did it correctly. For each unique set of Custom HTML pages (6 in all) you would create a New set, edit all the pages in that set, make sure the changes are saved and apply it to your list on the List Settings page.
More than one list can share the same set of Custom HTML pages by choosing the set from the List Settings page.
Please let me know if you are still having trouble with this.
Regards