The Act 1 Prelude – an evocation of The Grail – was played with the curtain down and the house lights half up, gauzy strings shimmering and beautifully gradated dynamics. Apart from a few moments of strangulated off-stage trumpets, the ROH brass was at its most disciplined, ebullient in the Act 3 Prelude. The Royal Opera Chorus was on magnificent form, jubilant at the sighting of the swan boat.