All done - in no small part to the help given here! I cannot for the life of me see the parsing of 1a) - to such a degree that i question if my entry is correct. However it fits with the rest of the puzzle, and the definition works.
Thanks to the setter, definitely a challenge, potentially too much cold solving needed, although in hindsight one solved, the clues arent all that impenetrable. Enjoyed the endgame, and definitely learnt something about cryptography.