Mystery and Imagination was a British anthology series in the 1960s, whose adaptation of Carmilla seems to have been lost. It aired November 12, 1966.
The screenwriter was Stanley Miller, who also adapted Le Fanu's Uncle Silas for the same program. Jane Merrow played the title character, with Natasha Pyne as Laura, Sonia Dresel as The Countess, Laurel Bathold as Mademoiselle de la Fontaine, Joseph O'Connor as the General (presumably Spielsdorf), and Aubrey Morris as Willibald (who, based on photo evidence, seems to be the Mountebank). Other characters, not recognizable by name, are listed.