John Kobal, 51, a British author and film critic who was heard regularly in the 1960s on the British Broadcasting Corp. radio program "Movie Go Round" which originated in New York. Among his many books were biographies of Greta Garbo and Rita Hayworth and such anthologies as "The Art of the Great Hollywood Portrait Photographers," "Hollywood: The Years of Innocence," and "People Will Talk," an oral history of the glory days of film. He had most recently completed an authorized biography of Cecil B. De Mille. In London on Monday.