Having managed the bottom half of this puzzle I thought I'd worked out how to treat the 21 words. However, I'm now totally thrown by the top left corner! If I treat 4d in the same way it seems to clash with my treated 15A. My 6D doesn't work any more either so please could someone tell me if I have the correct answer to 15A. My untreated word starts with P and ends in MI.
Sorry, Quizmonkey. I meant 2D, not 4D. I have the spare letter from 4D as part of my author's name! My brain has blown a fuse! Your help with 6D is greatly appreciated as I was trying to get the 3rd letter of that person's name from 5D. I will now persevere with that corner!