When Western musicians add their bit to traditional African music it can be disastrous: a programmed beat awkwardly forcing sinuous, sensual music to conform to its rigidity, or some dreadful rock vocalist doing a Bono all over some exquisite interplay of mbira and talking drums. But here we have a London collective working with a bunch of musicians from Nairobi, and refreshingly their presence doesn’t for one moment seem unnatural or intrusive.