Luther polarises opinion like a Marmite commercial starring Russell Brand. To some, it’s the only home-grown series that can hold its own against European crime heavyweights like Spiral and The Killing. To others, it’s a car crash of cop clichés; only the herculean efforts of Idris Elba in the title role holding together a show which looks stylishly gritty on the surface but lacks substance where it counts – all woolly grey overcoat and no Kickers.