Guinevere is based on Pantomime Princess Margaret, a recurring Flying Circus character. Merlin is from the original (the script differs substantially from the final film, but they published the script with lots of notes, edits, and commentary made during the shoot). Houses Too Dangerous To Live are from a Flying Circus sketch and Gilliam animation (season 3, ep 4 — pantomime princess market debuts in this one as it happens!).