Well that was frustrating. I spent most of yesterday going round and round in circles trying to understand a complex bit of decoding logic, before eventually finding the mistake on the schematic which was making all my subsequent logic terms wrong.
Thanks Acorn Computers 😩
@mfraz74 there's a good possibility I'm going to re-draw the whole thing. It's a very poor scan so I already had to draw in some illegible component and pin numbers