Codes, ciphers, enigmas

The Listening Service returns to its regular slot now the Proms are over, and chooses one of the BBC's "Ten Pieces III", Elgar's "Enigma Variations", to look at codes, ciphers and hidden messages in music. What might be the "dark saying" or mystery tune that the Enigma Variations are based around? Which other composers were keen on the use of codes and ciphers in their music? And if we can't crack the codes, does it matter? With Tom Service and Prof. Marcus du Sautoy.