Most sports documentaries share a format. It goes like this: from the raw material of latent talent is crafted, by means of discipline and despite various setbacks, a triumph. In this glorious conclusion, tears may be shed. Dark Horse: The Incredible True Story of Dream Alliance is giving away a fair amount from its title alone. Yet, this story of an unlikely Welsh National winner bred in the Valleys charms precisely by its surprises. It’s not an effect confined to Britain, either. The film won the Audience Award for World Cinema Documentary at the Sundance festival earlier in the year. Director Louise Osmond has employed conventional enough techniques – interview, archive and reconstruction – to tell the story. Yet as with her last cinema release documentary Deep Water