Something I didn't realize early-on was that Organon, the sooth-sayer, was played by Geoffrey Bayldon. As it happens, he was considered for the part of The Doctor-- back in 1963. He turned it down because, he said, he was tired of playing "much older characters". Several years later, he was in the running to play The Doctor in THE FIVE DOCTORS-- but JNT nixed it, because-- and I find this utterly absurd-- he felt viewers would too closely connect him with the character he played opposite Jon Pertwee on WORZEL GUMMIDGE. (You can also see the two of them together in THE HOUSE THAT DRIPPED BLOOD, it's Bayldon who sells Pertwee "The Cloak".)
You can see an example of his playing older characters in the Hammer HORROR OF DRACULA, where he plays the hotel manager opposite Peter Cushing. Cushing was much older than Bayldon, yet in the scene, Bayldon LOOKS much older than Cushing! (Cushing, of course, would go on to play his own version of "The Doctor"-- heh.) Both Cushing & Bayldon were also in FRANKENSTEIN MUST BE DESTROYED, but didn't have any scenes together. Bayldon played the "Police Doctor" who was browbeaten by Thorley Walters' abrasive Police Inspector. By rights, those two characters SHOULD have shown up for the climax of the movie!
According to Wikipedia this was the story from the season that was made first. I think Lalla Ward was imitating Mary Tamm's performance as Romana for at least part of it.