When Mavis Staples opens her mouth, remarkable things happen. She hollers, she sobs, she moans, she swoops, she rasps, she croons, she shudders. Not since I last saw Al Green have I seen such a bravura display of vocal prowess. On a rainy night in London, the 71-year-old singer of gospel, soul and R&B was in town for a one-off gig (and also for a Jools Holland appearance) to promote her new album, You Are Not Alone.