I seem to have solved this without really knowing how. I've completed the grid and found the 5 theme words and the 2-word phrase which describes them, all of which seem very obvious, but I have not knowingly made the "one update" which the preamble states is required. If anybody has completed this one, could they shed any light? I'm guessing it may be something to do with 11a, which IMO can only be one word (one of the thematics), but whose wordplay makes no sense to me.