You know that dream where you promise yourself you’ll play at next year’s British Horn Society festival, only to realise you actually have to go through with it and next thing you know your name’s on the bill between the society’s President and the horn section of the Berlin Phil.?

No? Oh. Well, Jasper Rees does.

Rees’s reputation as the Nick Hornby of the French horn strides towards musical infamy with his stage adaptation of I Found My Horn, the chronicle of an insane decision to perform, solo and under-rehearsed, in front of the nation’s most gifted practitioners of “the orchestra’s most difficult instrument”.

Worse than a rank beginner, Rees has horn form. Aged 17, he took a dive while playing in a school concert (literally: his performance was so awful he pretended to faint) and hasn’t picked it up in the intervening 25 years. But now he is on a quest, pricked into action by the failure of his marriage and the onset of middle-age.

Under the direction of Harry Burton, and unfettered by any strictures of realism, the one-man format, with its few props, unfussy lighting and quick costume changes, is ideally suited to the intimacy of the Tristan Bates.

Jonathan Guy Lewis (also co-author) is hilarious in each of about 17 roles, from the 15-year-old Rees to the towering figure of German hornist Hermann Baumann, via Mozart’s cheesemonger and Britain’s own inimitable Dave Lee: “If it feels like you’re trying to take a crap, that’s probably what it’ll sound like.”

A master of comic rhythm (and pitch: see below), for 75 minutes Lewis runs the gamut of emotions – pant-wetting fear, adolescent distraction, high-octane self-loathing and wistful, even teary, rapture – as he charts Rees’s life-affirming ‘progress’ from loser to nothing-to-loser.

And as a former hornist himself (modestly, in the programme, “a lapsed horn player”; but he nearly went pro), he demonstrates quite a talent for playing badly-on-purpose. In fact, my only reservation about the show is that, towards the end, the narrative thrust inevitably demands several consecutive minutes of genuinely competent playing! Still, that’ll be a great excuse for a montage when I Found My Horn transfers to the big screen.

n The show is on at the Tristan Bates Theatre, London W2, until December 20. Tickets: £11 (£9 concs.), telephone 0207 240 6283 (