"Fairs are becoming extinct," wrote historian Thomas Frost back in 1874. "The Nation has outgrown them and the last showman will soon be as great a curiosity as the dodo." A century on, there are still 150 fairs a week in the UK. Clearly, his grim prophecies have failed to transpire.

"Everyone says fairs are dying out but they never are; we just get too old to go to them," says Vanessa Toulmin. Her family go back five generations as showpeople; Toulmin grew up spinning candyfloss and operating rides and wrote a PhD on backslang, the showpeople's substitutions and codes that "flatties" (non-showpeople) can't comprehend. While writing her PhD, showpeople "just kept giving me stuff". Her collection now numbers 100,000 items. This forms the National Fairground Archive, which Toulmin has used to create an exhibition, Pleasurelands.

Walking into the exhibition, Susan Peak sighs: "It takes me right back." She can trace back eight generations of showpeople on both sides, and for her, even the blurriest photographs are full of stories. In fact, showpeople seem almost not to need an archive: a single name can spin Peak off into genealogical detail that would flummox researchers.

Her great-grandfather was one of the Grays family, which still runs the fairs on the Vale of Health in Hampstead, north London. His famous lodger Stanley Spencer captured the fun of the fair in his painting of the Grays' helter-skelter, which the showpeople's newspaper, the World's Fair, praised for the way "every movement of the happy Cockneys and every colour and outline on the famous skelter is portrayed with sincerity and beauty".

Admiring a gorgeously gaudy set of carousel horses, Peak identifies their lineage, going back half a century. "The modern look to me is garish, with all the pop stars painted on. I like the old ones. The really old gallopers used to have real horse hair and manes and they would have the names of the showman's children painted on."

Peak's maternal grandfather, Alf Stewart, ran boxing booths. "His fighting name was Spider Stewart. He had six daughters and he taught them all to fight." He also ran a one-pole circus where Peak's mother did a trapeze act. "She used to do bareback riding, jumping from one horse to another, doing somersaults. She was an extra for the Hammer House of Horror and she used to stick cornflakes on her face so it looked like she had smallpox."

Peak's father also fluttered with film, and was the original Captain Birds Eye on the TV ads. In between, he ran sideshows, including a Sleeping Beauty Show - a glass case exhibiting a barely dressed woman. "One day he was at Newcastle and she ran off with one of the boys on the dodgems," recalls Peak. "So he pulled back the sheet on the tent and he painted a sign on the front of it quick: 'Newcastle by starlight.' And he charged them to go and look at the sky."

Stewart ran sideshows, too. Fairground-goers assumed that "Hairy Mary from Borneo" was a real woman; in fact, says Peak, she was a monkey. But there were real freak shows at fairgrounds: Tom Norman, "The Silver King", was based at Croydon and is part of the Pleasurelands exhibition. He managed the Elephant Man as well as Mary Anne Bevan, "the World's Ugliest Woman".

In his autobiography he suggested that it was spin that sold the tickets: "You could exhibit anything... yes, anything from a needle to an anchor, a flea to an elephant, a bloater you could exhibit as a whale. It was not the show; it was the tale you told." It did seem to be a sellers' market; fat women shows (surely not too hard to put together) were so popular that in the 1890s there were five at Hull Fair alone.

Animal acts were another big draw. Peak's mother had a flea show. "She used to have an advert which said, 'Wanted: large live fleas.' She'd pay threepence for them. You looked through a magnifying glass and they'd pull a buggy or walk a tightrope."

Her father had a mouse show, along similar lines. "I had to look after them. You'd go in one day and there'd be 40 and the next day there'd be 70." Her grandfather, meanwhile, had an alligator as a pet. "They only grow the size of the container they're in: he had it in a bath, so it wasn't too big. He used to put a collar on it and walk it."

In the 1880s, wild west shows became all the rage, inspired by the American showman Buffalo Bill. "It's like Elvis impersonators today," says Toulmin. "There were lots of Buffalo Bill impersonators, but Texas Bill Shufflebottom was one of the most famous. He had 10 children and they all had wild west shows."

Shufflebottom toured his cowboy act for 60 years. This included knife-throwing, the so-called "Indian torture act", in which he aimed tomahawks within inches of his flinching daughters. "They weren't sharp," says Peak. "The board she was leaning on was papier-mache, so the tomahawks would go straight through and even if they hit her, they wouldn't hurt her."

Still, it was a tough life. "You do an apprenticeship from the day you're born," says Peak. "I knew how to make candyfloss from when I was four years old." The family travelled "from Land's End to John O'Groats, all over" with an amusement arcade, an archery stall, roundabouts, candyfloss and hot food stalls ("My dad was the first man in England to have a travelling hot dog stall").

In the winters, she and her brothers would go to school. "I went to 25 different schools. It was very hard for us. There was a lot of prejudice and bullying. I learned about life, I learned about people, and that's the best education anybody could have; much better than the three Rs, which I'm still learning at 52."

Peak's brothers are all still in the business, as are her children and husband who run a fast-food business. But fairgrounds, she says, aren't what they used to be. "We've come into the 21st century with a bang. All our rides are computerised, and the painting's airbrushed and modern. Where it used to take a week to build these rides up, we can have a whole fairground built up in two hours because everything's at the push of a button."

Showpeople have always been quick to use technology - when Thomas Bradshaw pioneered his steam-driven merry-go-round in 1861, a terrified reporter from the Halifax Courier declared: "The wonder is the daring riders are not shot off like cannonball, and driven half into the middle of next month." Although the older rides, captured at Pleasurelands on flickery black-and-white film, look pretty tame, Peak points out that "our rides in their times were thrill rides".

The walls at Pleasurelands are ringed with slot machines, with fat Edwardian pennies to make them go. One of the later models is called The Personality Test. "We call that a love machine," says Peak. "You put your penny in and whatever lights up is what you are: you're 'kissable' or 'domineering' or 'flirtatious' or 'hot stuff'."

Toulmin says: "The only thing missing from the exhibition is the smell of the candyfloss and the diesel and the green fields." When Peak gets nostalgic, she goes back to the fairgrounds and feels instantly at home. "It's a feeling inside, like a love affair. Your heart gives a little flip. The smells, the lights. It makes your whole body come alive. When you settle down you miss it terrible."

· Pleasurelands: 200 Years of Fun at the Fair is at the Croydon Clocktower until September 5. Details: 020-8253 1030.