Was Rossini, credited with the unsinkable comment that Wagner had "beautiful moments but bad quarters of an hour", hoist by his own petard in his last and grandest opera? For while Wagner, at least in performances as well-paced as the one I heard of Siegfried in the hands of last night's valiant field marshal Dominic Wheeler, really ought to have no dull moments, Rossini's Guillaume Tell offers many stunning quarters of an hour but just a couple which are so-so. In Chelsea Opera Group's blazing concert performance, however, none of it was less than compelling.