Over two ninety-minute episodes, the documentary “Jimmy Savile: A British Horror Story,” portrays exactly what its sub-title portends, the triumph of an odd but talented showman to reach the heights of British society (aka a knighthood) over nearly five decades, followed by sickening revelations, hundreds of them, of sexual abuse at his hands. That the rout of the Jimmy Savile legend did not take place until after his death is a blot on British society. Director Rowan Deacon is not showy but patiently choreographs archival footage with the testimony of Savile’s work-based acquaintances and dogged reporters. An intriguing subplot, as it were, seems to be that the monster revealed himself in public footage time and again, while cleverly deflecting real revelation. One can only watch so many true crime documentaries depicting humanity’s capacity for evil but Jimmy Savile, grindingly dispiriting as it is, is probably essential viewing.