You think you know Antonín Dvořák’s music quite well when you are aware not just of his greatest hits but also of the fascinating early symphonies and the dear early string quartets. When you know not only the moving Stabat Mater but also the prickly Requiem; and not just the cello concerto but also the violin and piano concertos. When, among his operas, you’ve seen not just Rusalka, but also heard (of) The Devil and Kate. And just then comes around the corner a work you maybe haven’t heard of – an embarrassingly beautiful one – that puts all your supposed Dvořák expertise into perspective: the oratorio Svatá Ludmila.