Old joke: when is N’Dour not N’Dour? When he’s Frank Sinatra. The comparisons of the Chairman of the Board with Senegal’s biggest star may seem a bit far-fetched, but I wondered as I watched him whether there’s a current European or American star who has the sheer authority, laid-back charisma and utterly distinctive voice that Frank used to have and Youssou has. In Youssou’s case, his voice of warm honey and mahogany is one of the seven wonders of the world. As it happens, for the first few numbers, Youssou was also as lounge-musicy as I’ve ever seen him.