In " Mrs. Harris Goes to Paris," a middle-aged woman of very limited means in post-World War II London dreams of owning a Christian Dior gown. In 1957, however, at the height of Dior's New Look renown, this was not a straightforward proposition. There was no London store or catalog to order from. There wouldn't even be a ready to wear line for another 10 years. Everything was custom made couture. The only way Ada Harris could own one herself, as the title suggests, was to drop in on the House of Dior at 30 Avenue de Montaigne.