Twenty Minutes - Stockhausen

German composer Karlheinz Stockhausen, who died in December 2007, was an icon of the avant-garde. Tom Service talks to Stockhausen's family, friends and collaborators - including Pierre Boulez, his son Markus Stockhausen, comedian and writer John Bird and pianist Nicolas Hodges - to find out just what it was that marked him out as such a pioneering and influential figure in 20th and 21st century music.