16a. Last one! Letters N-E-T. Have "write" up as "net" and last two letters as "information" "IT" giving NEPIT, which W'PDIA explains as a mythological being. Can't understand the clue. Explanation - or correction, anyone?
Thanks greatly BigDave44. Like Carmalee, I would never have sussed such a "deep" clue. I must admit, too, I never came across a "nepit / nat / nit" during years of teaching maths.