Andy, I am not sure I fully understand that, so let me state it as I see it.
You want the 'To Cover' items to have DV, that was expected, but what list should be in that DV? Will it be the same people for that site, everyone else, or some other list?
As to the DV functionality, it will only appear in the list if you put a new team member in the right place. So, if Bournemouth gets a new person, you need to add the person to the staff table, in a row where the other Bournemouth team are, and add the site id (you do not need the fullname, formula derived). You should insert the new person in place so that the team appear in the order you want. You cannot just add it at the end, the team need to be together. Do that, and that person will appear in the DV for Bournemouth- I just tried it and it works :smile:.