I finished the puzzle much earlier today, having struggled, like a few others, with 16 and 23 but finally managing to crack the logic behind these clues.
The only one I still cannot parse is 21. No one so far seems to have mentioned this clue, so either (a) I have the wrong solution OR (b) the solution is correct and I am the only one who can't parse the clue... A gentle hint may just be enough to nudge me towards the elusive PDM.