The Borgified Corpse wrote:
"State of Decay."
It's an interesting premise but feels a bit too thin to be an entire 4-parter. And, considering it's a classic horror pastiche, where's Robert Holmes?
He commissioned it in the first place, while he was still script editor and was busy writing Talons of Weng-Chiang!
It was going to be the first story of Tom Baker's fourth season, but the BBC was making a Big Important Version of Dracula, and insisted that Doctor Who couldn't do vampires that year as a result (Count Dracula starred Louis Jourdan, from Octopussy, and was written by Gerald Savory, who has a part in the tangled backstage history of The Celestial Toymaker).
So The Horror of Fang Rock was written as a last minute replacement (or maybe pulled out of Terrance Dicks's unused scripts drawer, to judge by Louise Jameson's memories of getting a first draft that called her character 'Sarah').
Three years later the unused script for The Vampire Mutations was in turn dug out of Terrance's filing cabinet, and rewritten for Romana, Adric and K9...