I once trod in an old Thomas Beecham joke about Karlheinz Stockhausen, with such a feeble punchline I couldn’t be bothered to remember it. Of course Beecham wasn’t the first classical musician of a traditional bent who found Stockhausen’s prodigious imagination too much to stomach – but when word went round the contemporary music community that Stockhausen was sending the Arditti Quartet up in helicopters there were some who said he was touched by genius. Others reckoned he was just touched.

A decade on, how does Stockhausen’s Helicopter String Quartet really stand up? Frank Scheffer’s gripping documentary follows Stockhausen and the Ardittis as they prepare for the premiere, and his film ends with the complete performance. Stockhausen is on hand throughout to explain himself. If the concept of sending each member of a string quartet up in their own helicopter, where what they play is beamed back into the concert hall and mixed electronically with the sounds of the ’copters seems eccentric, then Stockhausen explains he is simply following the directions of his dreams. From Gruppen – his 1957 piece for three spatially distributed orchestras – onwards he was always interested in the idea of sound flying around the heads of his listeners. And here is a supreme realisation of that fantasy.

You could argue that Stockhausen had long since lost touch with reality by the time he conceived and executed this piece. But that’s missing the point: although Stockhausen’s rock-star ego did lead him to produce some tedious late pieces that relied on the unquestioning faith of his groupies, the Helicopter String Quartet has a charm and fascination of its own. As Stockhausen deconstructs his material during rehearsal, singing bits to the players, it becomes clear there is real harmonic depth and melodic direction. Shame about the carbon footprint though.