Surtitles at the English National Opera? Perfidia! Aiuto! Aiuto! ("Treachery! Help! Help!"). You may recall that, about 18 months back, ENO found itself at the centre of a brief brouhaha. It was reported - incorrectly, as it turned out - that the company, which always sings in English, was planning to introduce self-invalidating captions at performances. It's therefore a nicely droll touch that Michael Frayn, in his new adaptation for ENO of Offenbach's La Belle Helene, not only calls for surtitles to be flashed up during the inset parody-operas that are a feature of his version, but also arranges for another, more mischievous innovation. At one point in the piece, the …