Finished, with a couple of uses of Crossword Solver and some nudges from the kind folk here.
FOI: 13a
LOI: 16. Never heard of him, but invented the name from wordplay and 4 crossers and googled to verify. The whole SW corner was very slow.
Cod: probably 12, though I needed a hint here about the first 2 letters before it fell into place
Can't parse first half of 20a.
I parse 9 differently from posters above:
Def=rubbish as verb
First 4 letters as in "....stage" "....premises" etc,
Then 2, 2.