Four letters: BACH. For an organist, they’re the beginning and the end – and the centre of the musical universe, too. Thomas Trotter has built this whole, inspiring programme around Bach’s magnificent Chorale-partita. First, though, Liszt and Schumann pay deeply romantic homage, and then Judith Weir’s poetic musical watercolour and Mulet’s cascading showpiece send us out in a blaze of colour. On the Symphony Hall organ, it should sound magnificent.