“Based on a true story” remains one of cinema’s most reliable tag lines. No matter how small the foundation, how loose the interpretation, the suggestion of veracity can burnish the dull, smuggle through the implausible and generally make audiences more forgiving of contradictions in narrative. With Philomena, no such indulgence is required. The film is based on fact crueller than most melodrama, it is played with respect and vitality (no easy combination) by lead actors Judi Dench and Steve Coogan and it concludes with a surprise that is not a plot device but a real revelation.The impetus for the film initially came from Coogan who saw a photograph of the former BBC foreign correspondent Martin Sixsmith taken with a woman, Philomena Lee, whom he had helped, in what bega