The gawky kid is wrestling now with one of those blobs of Saint-Saëns that sound like sticks of candyfloss with added sugar. His mousy-looking mum is at the piano and his tone is unsteady. Marks of six, seven, five, thank you and good night. Yet, back a track, at a certain point in the Bach, the boy did something that almost did for me. If he does not repeat it in the French petit-fours, I’ll make him play as long as it takes to find out what it is.

As the French froth dies away, I make a sedentary gesture to the other judges and, exercising the chairman’s prerogative, call for an extra piece. ‘You sure we need this?’ scowls Olly. Fred Burrows looks bewildered. Peter Stemp, bemused, whispers, ‘Johann Sebastian Bach: Sarabande from partita number two in D minor’ and within a minute I know all I have ever needed to know about Bach, the boy and the meaning of music.

Playing Bach on the violin is not child’s play. Although the music is generally easy on the ear, the partitas and sonatas are pitted with twists and finger-stretches whose difficulty is masked by the simplicity of the sounds produced. A player can bring off the most dazzling leap without anyone but a fellow-virtuoso knowing what he has achieved. Should he slip or miss a note, however, the least musical of listeners will sit up and spot the error.

Having got the piece under his fingers, the fiddler has then to release the music from its formal chastity-belt and reveal its hooded secrets. The art of interpreting Bach is a book of seven seals that only immortals have unlocked. Bach is not meant to be played competitively. But the transparency of the solo violin works is a gift to competition judges, for no other music so readily separates the meritorious artist from the meretricious.

To bring this music to life, the soloist must overcome the inertness of notes and infuse a gust of his own breath, like a paramedic performing emergency resuscitation. The crotchets and quavers are printed blobs on stave, to be played exactly as written. There is no licence for improvisation or messing around. The only way for a performer to express himself within the strict traditions of classical music is by shading the relative values of the blobs – shaving a nanosecond off one note and adding it to the next.