You are required to have a drivers side wing mirror and either an internal mirror or a passenger side wing mirror. "Any mirrors additional to the obligatory mirrors are not subject to this inspection"
http://www.ukmot.com/6-6.asp
So as long as it's the near side mirror damaged and it's secure and doesn't have any jagged edges likely to hurt pedestrians, (which could cause it to fail in another section) then it's not tested.