I know I'm very late to the party but I'm almost done and need a nudge please...
Solved all the clues except 18d and, despite having read all the helpful comments about how to tackle the coding I'm stumped. I see the repeated letter in 20ac which could make a possible food but other contributors have mentioned that the code is pretty random, in which case I can't see how to proceed unless each letter shifts by the same amount each time (as bobbycollins mentions in #84).
Pretty frustrating to have got this far and fallen at the last fence so any hints would be gratefully received!