Alexander Armstrong and his Band Celebrate the Great British Songbook, St James's Theatre, review Hugh Laurie did it. Rob Brydon does it from time to time. And now Alexander Armstrong is at it as well. There seems to come a moment in the midlives of the more reflective sort of comedian when they begin to wonder what might have been. Next thing you know, they've burst into song. Armstrong's debut at the inaugural London Festival of Cabaret is an engaging and very British cocktail of dry humour and wild eccentricity, spiked with the faintest tinge of melancholy. His four-piece band, dressed in dark suits and white shirts, each with a poppy in his buttonhole, look vaguely menacing - as though the Reservoir Dogs had decided to pay their respects at the Cenotaph. Armstrong, in the same get-up, looks like a mild-mannered GP (unsurprisingly, since the Pointless presenter is descended, he confided, from nine generations of doctors). There was the odd medical issue on stage, as it happens. His pianist, who works under the nom de guerre of "Harry the piano," was playing with a broken wrist, which he held up pathetically for a chorus of audience sympathy. Armstrong, meanwhile, was struggling with an intractable cough, which clearly bothered him, but didn't seem to affect the power of his burnished bass-baritone, or the rollicking energy of his celebration of British songwriters. His selection of classics from the Great British Songbook was wildly eclectic, beginning with a chilling a capella rendition of a doomy Northumbrian folksong in a dialect so impenetrable that he might have been singing in Icelandic. There was a nod to his classical training (he was a music scholar at Trinity College, Cambridge) with a haunting off-mic account of Benjamin Britten's O Waly Waly. Who knows what Britten would have thought of finding himself up against Prefab Sprout's Hallelujah, Chas and Dave's Ain't No Pleasing You and Mike Batt's Wombling in the Rain. Probably he would have appreciated the sheer oddness of the emotional range, not to mention the dextrous musicianship of Simon Bates on woodwind, Pete Cater on drums, Dave Swift on bass and poor old Harry, whose busted wrist didn't inhibit a virtuoso improvisation on tunes requested by the audience. An aseptic rendition of Bowie's Changes was a rare low point, swiftly erased by the sight of the notably staid audience bopping along to Armstrong and his band as they let rip with a beguilingly bonkers encore of Cyndi Lauper's Girls Just Wanna Have Fun. Until October 31. Tickets: 0844 264 2140; stjamestheatre.co.uk