Ask a roomful of concert pianists to pick their graveyard moment in Rachmaninoff’s Piano Concerto No. 3 (1909) and they’ll almost certainly point to five or so pages halfway through the last movement where an ant nest of piano notes infests a sparse orchestral threnody. When an elderly Vladimir Horowitz performed this passage – lank, dyed pageboy hair framing his Bela Lugosi face, hands darting over and under each other like butterflies – he looked more like a weaver at his loom than a virtuoso at his instrument.