Crazy plots and preposterous characters are at the heart of Gilbert & Sullivan's enduring appeal, says Michael Church

Arthur Seymour Sullivan and William Schwenck Gilbert didn't like each other, didn't have much in common, and both had loftier ambitions than the creation of operettas. But if they'd stuck to those ambitions, they wouldn't be remembered today.

Sullivan was hailed in his youth as a budding English Mozart, but despite his dreams of greatness never lived up to that promise. As a composer, tout court, he's best known today for "Onward Christian Soldiers"; cognoscenti may recall him as the composer of minor choral works, plus that quintessentially Victorian weepie "The Lost Chord". Gilbert was a ferociously prolific journalist, versifier, and playwright, but if that was all he'd ever done, his name would have been long forgotten.

Gilbert and Sullivan were aptly described as "whisky and soda", and impresario Richard D'Oyly Carte was their tumbler: he wanted to create an English school of comic opera to rival the Parisian operettas of Offenbach, and instinct told him the fusion of this unlikely pair might do the trick. One snowy morning, Gilbert was persuaded to call on the starry young composer with a new libretto he'd written called Trial by Jury. Gilbert read it out loud with a bad grace, as though the whole thing was beneath him, and finished with a contemptuous snort: he hadn't noticed that Sullivan had been rocking with silent laughter throughout.

Within a fortnight Sullivan had set it to music, and it became an instant hit in a Dean Street theatre. It transferred twice and ran for two years: time, thought Carte, for another, hence The Sorcerer, which did even better. Now Gilbert couldn't stop: a few months later he sent Sullivan the manuscript of HMS Pinafore, which sparked a commensurately rapid piece of composition, despite the fact that Sullivan was in acute pain from the kidney disease that would plague him.

It may interest Jo Brand to know that her lack of musical or dramatic training was exactly what the choleric Gilbert – who controlled every detail of his productions – looked for in his performers. He regarded his libretti as a composer does his symphonies, where a false note can wreck everything: every word had to have a certain inflection, every gesture had to be choreographed, and he worked out each scene with wooden figures in a model theatre on his desk. He hated prima donnas and "tenors"; when a senior actor complained, after dozens of repetitions of a particular piece of business, that he'd "been on stage long enough", Gilbert brusquely agreed and sacked him. He didn't want stars: he wanted actors who could work as an ensemble and communicate the raw pungency of his lines. George Grossmith Jr, whose incarnation of the blustering Sir Joseph Porter in Pinafore was universally acclaimed, had previously been employed as a reader of religious tracts for the YMCA.

Sullivan's way of working was to compose the choruses first and the quartets and trios next, leaving the duets and solos to the last moment, which added to the performers' first-night nerves. But he had nerves, too: "Light and airy as they may seem," he told an interviewer, "but my comic operas give me far more trouble and anxiety than a cantata like The Golden Legend." And this is a pointer to the brilliance of what he achieved in these "Savoy" operas, which took their collective name from the theatre where they were produced.

And brilliance is the word. As a composer in his own right, Sullivan's musical voice was merely tasteful, but Gilbert's fanciful wit turned him into a cornucopia of melodic and harmonic invention. As a student in Leipzig, Sullivan had become enthused by Schumann and Schubert, and the beauty of his part-songs and the subtlety of his orchestration bear constant witness to this. His word-setting for Gilbert's tongue-twisting lyrics was miraculously apt, and his rhythmic ingenuity inexhaustible; as the drama hotted up, he often set two or three melodies going at once.

The snobbery "G&S" still elicits from supposedly sophisticated people is pure prejudice: as music theatre, the Savoy operas are up there with Offenbach, and have infinitely more bite. When the Arts Council of Great Britain cut the D'Oyly Carte company's grant in 1982, claiming the whole thing was insufferably fusty, they killed a performance tradition which had continued for 90 years, and thus destroyed a priceless link to the source.

It's no surprise that Jonathan Miller – one of our best opera directors – should have chosen The Mikado as his vehicle for two hours of dazzling theatrical invention, or that his show is still packing the Coliseum after more than two decades (and getting its umpteenth revival on Saturday. Nor should we wonder that, aided and abetted by composer Carl Davis, Mike Leigh should make this opera, and the Victorian Japan-mania which gave rise to it, the basis for his exhilarating film Topsy-Turvy. Nor that Joseph Papp should have drafted in a posse of showbiz celebs, led by Linda Ronstadt, for an acclaimed production of Pirates in New York's Central Park. Really sophisticated people have always got the point.

Was Gilbert a satirist? No, says Leigh. Yes, say others, a latter-day Aristophanes, with Lord High Executioner Ko-Ko's perennially updated "little list" only a bit of it. Consider the pluralist grandee Pooh-Bah, who combines the offices, inter alia, of archbishop, lord mayor, paymaster general, and lord chief justice: puts one in mind, does it not, of a recently disaffected Blairite?

Consider Sir Joseph Porter, who proudly explains how, by polishing brass door-knockers, serving writs as a clerk, and getting filthy rich, he assured his entry into Parliament and beyond. Those who wish to emulate him, he sings, must follow his golden rule: "Stick close to your desks and never go to sea/ And you all may be rulers of the Queen's Navee!" New Labour couldn't put it better. Gilbert's libretti are crammed with such timelessly pertinent aperçus.

But the real joy of Gilbert and Sullivan lies in the way the crazy logic of the plots and the preposterousness of the characters are magically fused through enchanting music. After 13 scratchily brilliant years, the dynamic duo parted acrimoniously, but their work is as fresh now as it was when the Prince of Wales and his mother Victoria – who was amused – led the applause.