Before this revival of the razzle-dazzle 1980 musical opened in the confined setting of the Menier Chocolate Factory, the director Gordon Greenberg said that his problem was how to stuff the greatest show on earth into London's smallest theatre.

Amazingly, as he tells the story of the great circus impresario PT Barnum, purveyor of curiosities such as the oldest woman and the smallest man alive, he has pulled that off. Paul Farnsworth's brilliant design turns the pokey space into an evocative big top, plastered with posters and full of the whiff of sweat and sawdust. Miniature dolls house models hang from the ceiling, or perch on top of props boxes, to tell us exactly where we are at any moment. Hung with multi-coloured lights (wonderful lighting courtesy of Philip Gladwell) the whole place whirls with noise and colour, as circus barkers, acrobats, magicians and dancers celebrate the wonders on show.

But then Greenberg forgot the most important thing: without a great showman, the circus ring is as empty as the hole in a donut. In casting comedian Marcus Brigstocke as Barnum, he undoes all the magic he has created.

It's not Brigstocke's fault that he can't rise to the role of ringmaster of this tuneful three-ring circus (music by Cy Coleman, lyrics by Michael Stewart and book by Mark Bramble). He tries desperately hard to pull it off, displaying an easy charm and, as soon as he is off script, he seems relaxed and at home. Asked to act and to sing, he is all at sea. He can't walk a tightrope as the plot demands, and although he disguises this with well-worked asides, it does sum up the problem. His performance is monochrome and under-powered where it needs to be huge, emotional and charismatic.

His shortcomings are all the more obvious because the ensemble around him is so exceptional. As Barnum's put-upon wife Charity, Laura Pitt-Pulford brings slivers of light and shade and a wonderfully expressive voice to a character who could easily be a one-note bore. Celinde Schoenmaker's singing positively soars as Jenny Lind; even more impressively she wrings every ounce of comic meaning out of the scene where she seduces Barnum with her heaving bosom and breathy Swedish tones. Dominic Owen is a bitingly impressive ringmaster. Harry Francis illuminates the space in his sharp-footed, high leaping turn as General Tom Thumb in "Bigger isn't Better."

But the very best of this Barnum are the characters who have barely a line: the singers and dancers who fill the arena with constant, teeming life and close-up feats of acrobatic wonder. We have a fire-eater, girls who can stand on a man's head with the same ease as they fly across the ring with high-thrown turns full of trust, suppleness and courage, men who can shimmy up poles in the blink of an eye. Marshalled by circus expert Scott Maidment and choreographed by Rebecca Howell they are marvellous and astonishing to behold.

The music direction by Alex Parker and the orchestra also sparkle. When the ensemble is in full, sweeping, vivid flow in numbers such as "Come Follow the Band" and "Join the Circus", the impact is irresistible. It is when Barnum is telling his story centre-stage that the energy flags. What a shame.

Barnum runs at the Menier Chocolate Factory until 3 March.