When I missed the only possible train two weekends ago, I thought I'd blown my chances of seeing the Met's La sonnambula for good. But fate was in my favour for once, and a glitch in that session meant the broadcast was re-screened last weekend. It meant going to a different theatre and buying a second ticket, but so what? Natalie alone is worth far more than that, and besides, I was determined to see this production. Anything which generates as much controversy as Mary Zimmermann's production of La sonnambula did on its opening night has to have something going for it.