The ending was to have originally had the vampires destroyed by a swarm of bats. This ending proved too expensive to stage and shoot. The concept of this ending was recycled three years later for the climax of Hammer's Kiss of the Vampire.
The film underwent rewriting by others including director Terence Fisher who made changes to the script on the set just prior to shooting scenes.
The prop department put a lot of effort into making a realistic model bat. It got lost and had to be replaced on short notice. This explains the rather unconvincing look of the model that got actually used in the movie.