Quite what it was that was so spellbinding about a quartet of middle-aged German blokes in skintight bodysuits standing at neon-lit consoles is difficult to articulate. They didn’t even seem to be doing very much up there on stage. But the audience of 1,000 for round two of Kraftwerk’s eight-night retrospective in Tate Modern’s Turbine Hall were mesmerised for two hours solid.