The anniversary show, Quack Oops! 20 Years of Viz, at the Cartoon Art Trust, honours a home-made publication which at its height could compete with anything on the shelves.

In the late 1980s it graced the schoolbags and briefcases of more than a million avid readers.

The family of Viz characters

Nowadays even the magazine itself is willing to admit on its front cover: "Not as funny as it used to be (and it's losing sales)".

Sales slump

Die-hard fans might take issue with the first half of the statement. But no one is denying that sales-wise Viz is now just a shadow of its former self.

A decade ago Viz was poised on the cusp of mega-circulation. Its potent recipe of profane language spouted by quirky comic book characters, complemented by a dash of tabloid parody and "crap" jokes had come of age.

From its humble inception as a photocopied rag with a print-run of 150, it won prime placing in almost every newsagent in the country.

Viz favourite: Roger Mellie - the Man on the Telly

Like Dennis the Menace and the Bash Street Kids before them, Viz's own roll-call of stars - Buster Gonad, Roger Mellie and Biffa Bacon - became household names.

Viz turned the conventions of the comic book on their head. Aquatic soccer star Billy the Fish - to be immortalised Damien Hurst-style in a glass tank at the exhibition - parodied the heroics of Roy of the Rovers.

Readers lapped up the bizarre antics of characters such as Johnny Fartpants, Sid the Sexist, Roger Irrelevant, Mrs Brady - Old Lady, Spoilt Bastard, Farmer Palmer, Big Vern and, perhaps the most infamous of them all - the Fat Slags.

And when that all got a bit too much there was always the letters page, where Viz would act as arbitrator in readers' disputes; and the Top Tips column - a hilarious rip-off of the handy hints section run in women's magazines (see below).

The brain-child of Chris Donald, who put the first edition together in 1979, with the help of his younger brother Simon, some friends and a Xerox machine.

By 1988 it was selling in the hundreds of thousands. The following year sales nudged one million, peaking in early 1991 at 1.1 million, just behind the Radio Times, TV Times and Reader's Digest.

Although Donald has relinquished editorial control, Viz is still reckoned to have average sales of 300,000 per edition. Readers also log on to the Viz website.

Despite the drooping sales, comic expert Kev Sutherland says Viz is still the biz.

"In a sense it's unfortunate that Viz did so incredibly well in the past because it will always be compared to the heyday.

Still selling by the truck-load

"Selling 300,000 copies is the sort of circulation that most comic or magazine publishers would kill for."

Comic historian Malcolm Phillips thinks it "touched a nerve".

"The humour was descended from the tradition of seaside postcards and that's a very, very essential part of British humour," he said.

His particular favourite was the sniggering schoolboy Finbar Saunders, whose acute ear could discern a double entendre in just about every sentence.

Still rated

Mr Sutherland is still one of the dwindling band of fans. "[The team] are astounding. I've never seen comic creators create so consistently."

And in his eyes, at least, Viz now works the old magic better than before. It's "rarely been better," he says.

Even if the glory days are over for Viz, according to Mr Phillips, it has secured a place in the comic hall of fame to rival more traditional titles.

Good investment

First editions of Viz have reputedly been sold for �485, matching the top price paid for a 1950 debut edition of the venerated boys' comic Eagle.

But now that the Viz bubble has burst, Mr Phillips thinks it will soon run out of steam.

"I don't think it will be around in 10 years time. People are looking for something else, something fresh."

The magazine may yet find a new following. Welcoming the new exhibition, The Daily Telegraph applauds Viz's portraits of "Left-liberal archetypes".

With characters such as the feminist Millie Tant and the hopelessly new age Modern Parents, the Telegraph argues Viz is in its element lampooning Britain under New Labour.

Likening it to the "iconoclastic Toryism of Swift", an editorial says: "Viz offers an honest subversiveness which a conservative newspaper can admire."

With such rousing support maybe Viz will be able to keep it up for another 20 years.