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The Secret Cave
Production Companies: Merton Park Studios, Ltd.; Children’s Film Foundation (UK). Distributor: Associated British Films. Director: John Durst. Producer: Frank Hoare. Screenplay: Joe Mendoza, based on Hardy’s novella “Our Exploits at West Poley.” Cinematography: Martin Curtis. Length: 62 minutes. Premiere: 1953.
Like Under the Greenwood Tree, The Secret Cave may not actually be lost; however, information on the film's availability is scarce. A children's film, The Secret Cave has the distinction of being the only English-language motion picture based on a Hardy source between 1929's Under the Greenwood Tree and 1967's Far from the Madding Crowd.
... The same story would be filmed again in 1985 under the name of the source material, Our Exploits at West Poley.
Subsequently typing "the secret cave" 1953 brings up a lot of links for the film, including the following:
If anyone remembers the popular childrens show "Animal Magic", this was presenter Johnny Morris's first film role.