Towards the end of her charming set at Cheltenham Jazz Festival, the British singer Corinne Bailey Rae paused to consider what genre her music belonged to. It’s a question that the festival has asked itself – and answered very effectively. Define jazz too narrowly and you end up with a room of 12 inveterate foot-tappers; go too far the other way and the term becomes meaningless. Now 25 years old, the Cheltenham formula – mixing Grammy-winners like Bailey Rae with standard-bearers and avant-gardi