It’s not often that a performance of Purcell’s King Arthur requires its entire cast of singers to strip down to very tight Union Jack boxer shorts. It’s not often either that the audience find themselves actively encouraged to talk over the music, yet both were unexpectedly and riotously true last night at the Spitalfields Festival. Pairing Baroque big-hitters The English Concert and I Fagiolini, there was nothing half-hearted about this semi-staging of Purcell’s semi-opera. It promised much and delivered more, and while those listening live on Radio 3 might have enjoyed better textural balance, they can’t have had nearly as much fun as the sell-out crowd sweltering away in Shoreditch Church.