Well I've almost got to the bottom of this now (after sabotaging the NE by mis-spelling 7d as I entered it, thereby messing up the otherwise easy-ish 11, 13 and 16!) I kind of see a thematic quality to some of the answers but nowhere near the 15 or 16 that others are discussing.
Just 20d and 28a left to get and I don't see either of them. Any tips welcome...