A large musical ensemble from across Wales come to the Royal Albert Hall to give the first complete Proms 2012 performance of Leonard Bernstein's Mass, a work that combines religious observance and musical theatre. Kristjan Järvi conducts the National Orchestra of Wales, the National Youth Orchestra of Wales and eight choirs of talented children and adult singers, with choruses of street kids and a rock group thrown into the mix. Introduced by Petroc Trelawny.