Men react to the arrival of middle age and the crises that come with it in very different ways. Some take up cricket or paragliding or marathons, some take up with late-flowering Lolitas. At the age of 43 the arts journalist Jasper Rees, finding himself facing divorce, depression and a house full of grungy boys mumbling “whatever”, decided to go forward into the past. He took up the instrument he had abandoned in teenage embarrassment 25 years earlier: that “16ft of coiled brass tubing”, that fiendishly cumbersome source of melodious raspberries, the French horn.

I Found My Horn is the thoroughly enjoyable monologue that he and Jonathan Guy Lewis have adapted from Rees’s printed account of his struggles with a horn he loves and mistrusts as if it were some wayward mistress. In Harry Burton’s spare production, Lewis plays both Rees and his horn, the first very well, the second as erratically as the text demands. There’s frustration and there’s a panic signalled both by the actor-musician’s first appearance — ashen, half-naked, his private parts hidden by that twisted metal — and by his quivering face as he prepares to tackle Mozart’s Third Horn Concerto, K447, at the annual festival of the British Horn Society. K447 is his personal K2, a Himalayan challenge, and, after a comically dodgy opening he manages to touch its heights or at least progress farther than base camp. What begins by sounding like murder ends up sounding like Mozart.

And that gives an agreeably upbeat ending to an evening that has seen the versatile Lewis play a lot more than his intractable horn: the smilingly reproachful schoolteacher who can’t comprehend his wrong notes; a genially blokeish musician and mentor called Dave Lee; the lumbering German genius, Hermann Baumann, who treats Rees with grumpy hauteur at a New Hampshire “horn camp” that comes complete with log cabins, musically eager nuns and only intermittent encouragement.

Rees himself comes across as nervous, insecure, eager, likeable and, at the end, joyous. He’s learnt to trust his horn and himself. His awkward affair with his instrument has become a marriage, and one that looks like having a future. Can you think of a better way of facing 43 and a dark-seeming future? Not many.

Box office: 020-7722 9301, to Nov 28

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010