So many fashion documentaries are frigid and vapid; this one, though, from film-makers Trish Dalton and Sharmeen Obaid-Chinoy, has a story to tell about someone with something to say. Diane von Fürstenberg is the designer, entrepreneur, former Olympic-level hedonist and Studio 54 habitué, who once (separately) got it on with Warren Beatty and Ryan O’Neal over the same weekend but airily turned down a threesome with Mick Jagger and David Bowie. She was also the daughter of a Belgian Jewish Holocaust survivor, Lily Halfin, to whom she was very close throughout her life and was devastated when Lily suffered a...