Michael Houstoun first performed the cycle of 32 Piano Sonatas by Beethoven 20 years ago. Why do it again? What's changed? Michael Houstoun talks to Tim Dodd about the project and they look at four sonatas from the first wave of concerts: the graceful Sonata quasi una fantasia in Eb Op 27 No 1; the tormented Op 31 No 2 in D minor, the "Tempest"; the expansive Op 53 in C, the "Waldstein"; and the gigantic Op 106 in B flat, the "Hammerklavier".