Let's try to be positive about this: you would pay a fortune for two hours of holistic vocal therapy anywhere else in the West End, so Bobby McFerrin’s London Jazz Festival show can probably be considered a bargain. He did not help my stress levels one bit, to be absolutely honest, but everyone around me was eager to give him a standing ovation at the end. Some people, I’m afraid, are so star-struck that they would hand over good money to watch Elton John go shopping.

There is no question that McFerrin, a walking synthesizer with greying dreadlocks, is a charismatic talent with an omnivorous appetite for music and a rare gift for communicating with audiences. While most of the world still remembers him as the man who gave us the irritatingly catchy Don’t Worry, Be Happy, he has built an unconventional career as a classical conductor, as well as a proselytiser — on albums such as Circle Songs — for the joys of a cappella ensemble singing.

For all that, there was a terribly lax quality to this display. McFerrin is very good at making car noises, among other things, but for most of this concert the vehicle’s engine hardly got beyond second gear. For much of the time he did not even seem particularly interested in leaving his garage, content instead to simply stand around polishing the bonnet and playing with the windscreen wipers.

The presence of the two-dozen-strong London Jazz Festival Choir added some variation to the repertoire of chest-percussion and mildly funky mantras. But compared with the delicate interplay on his studio work, the choral pieces sounded rather tentative and under-rehearsed. It was as if we were listening to warm-up exercises rather than the finished product.

McFerrin soon brought the audience into action, asking them to carry the melody of Gounod’s Ave Maria while he sang the ever-popular C-major prelude from the Well-Tempered Clavier. Full marks to the people in the balcony.

Time and again, though, McFerrin’s own contribution did not rise much beyond the level of a party trick. That invisible synthesizer too often sounds like a budget keyboard from Dixons. He surely needs the challenge of stronger material.

He turned himself into a double-bass in a short series of duels with a guest trumpeter. Dancer Sheron Wray went free-form on a couple of numbers that included the bluesy standard Walkin’.

At the encore, McFerrin launched himself into a potted version of The Wizard of Oz, complete with a water-shrunk wicked witch. His children must have a wonderful time when he reads them a bedtime book. But most adults, unless they happen to be Harry Potter fans, need something more grown-up.

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