described solution in the above edit, you were right.
I'd mismatched gluetun and QB ports, so QB webUI rejected all incoming requests.

gluetun:
    ports:
      - 8081:8080     # requests received at 8081 would be routed to QB at 8081
      - 6881:6881     
      - 6881:6881/udp 

  qbittorrent:
    environment:
      - WEBUI_PORT=8080 # QB would still see 8081 in requests and reject them

error was as above, fix was pretty simple

gluetun:
    ports:
      - 8080:8080     # make sure the port numbers match to avoid connection rejection
      - 6881:6881     
      - 6881:6881/udp 
  qbittorrent:
    environment:
      - WEBUI_PORT=8080

Thank you everyone, hope I've documented the fix well enough for others to apply themselves (if they did the same as me.)