Amid a steady diet of doom and gloom – From the House of the Dead to close out last season, and Norma to open this one – the National Theatre served up a palate freshener with a cheery double bill, Stravinsky’s The Nightingale and Tchaikovsky’s Iolanta. The latter proved to be the crowd-pleaser on opening night, but the opener was the real star of the evening, dazzling in its inventiveness and visual allure.