You saw the ad and bought the dress. But what about the hatchback?

Meg Carter looks at the spin-off industry created by cult commercials

Unbelievable but true, a dress worn in the latest Gold Blend TV commercial was sold for pounds 2,500 at auction in June. Meanwhile Harry, the angel faced toddler in the Safeway advertising campaign, recently launched his own range of tea-towels, mugs and badges. And Terry, star of the Pot Noodle ads, has his own fan club with the obligatory range of merchandise due out this autumn.

Proof, if any were needed, of the pervasive power of advertising. But make no mistake, the 'cult' status enjoyed by a growing number of ads is no accident. Advertisers are finally waking up to the potential of selling us not only their products, but a range of campaign spin-offs, too. Insiders call it the "Levi's effect" as each time the jeans brand launches a new commercial, the soundtrack inevitably ends up riding high in the pop charts.

Certain ads have always caught the public's imagination. When Volkswagen ran a TV commercial featuring a young woman apparently driving to a wedding (in fact, she was just about to get divorced), hundreds of women inundated BMP DDB Needham, the agency behind the campaign, with requests for VHS copies. The ad's success even persuaded pop group The Bluebells, whose song 'Young at Heart' featured in the ad, to re-form.

Meanwhile, another VW commercial - featuring a small girl being driven through the mean streets of a big city - was immortalised in greetings cards and posters by Athena. Sales were apparently fuelled by mums eager to show hairdressers how to re-create the blonde, ringletted look for their own little dears.

Sometimes, even the most peripheral minutiae can catch the public's eye. Sales of a pair of Tiffany earrings (pounds 85) worn in one Gold Blend ad soared after the commercial broke on air.

It was only a matter of time before some sharp suit decided to cash in. Amongst the first was the fashion industry. Debenhams reproduced the Just Divorced girl's hat for under pounds 30. When Nicole, the French gamine in the Renault Clio campaign, appeared in one ad wearing a scoop-front spotty dress, a high street craze was born. "We were inundated with calls," says Mark Robinson, new business director at Publicis, Renault's ad agency. Luckily, Miss Selfridge came out with a similar number at just the right time: the frock sold at pounds 19.99. "We referred all callers to their local branch."

Then came coffee. Gold Blend's 'will they, won't they?' romantic saga has spawned a book, Love over Gold, which reached Number 4 in the charts; a compilation video; two CDs (pounds 17.99 and pounds 12.99) and the Alternative Euro 96 London film season (for the "soft at heart"). "We decided to advertise it like a soap and promote it like a television programme," says Jerry Green, creative director at McCann Erickson which created the campaign. "It's golden publicity for Gold Blend."

And now snack products. Following their success with Gary Lineker, Walker's Crisps has launched a new flavour: Salt'n'Lineker. (Rumours of plans for a second - Tears'n'Onion in honour of a subsequent ad featuring a weeping Gazza, remain unconfirmed). Meanwhile, Golden Wonder has launched a range of merchandise and even a fan club based on the characters in the current Pot Noodle campaign. "Our entire approach is for brands to provide an experience for the consumer in as many ways as we can," explains Steve Henry, a partner at HHCL, the agency behind the Pot Noodle ads. Each features a battle of wills between a character called Ned Noodle (who dresses up as a Pot Noodle) and Terry - a man from Pontypridd who campaigns against Pot Noodle's "fibrous lies".

So far, the range of Ned-inspired promotional products, includes a wall clock (six Pot Noodle lids plus pounds 7.99), fridge magnets (six lids plus pounds 1.99), and a tea-towel (five lids plus pounds 1.99). A bendy latex Ned will be launched in October. And Terry has his own club: the League Against Fibrous Lies, with a membership pack and campaign materials available to whoever writes in.

"Producing ads in a vacuum is pointless," Howell says. "It's too expensive to do communication that doesn't last." Spin-off merchandise produces a lasting reminder that sits on the desk or in the home. Back in the Seventies it worked for Fred, Homepride's graded grain man. Once immortalised in plastic, replica flour shakers quickly infiltrated kitchens across the land. And now Tango, another HHCL client: "So far, we've sold 250,000 orange rubber men."

According to industry folklore, when battery manufacturer Energiser launched a US TV campaign featuring a drum-banging pink rabbit (subsequently know as the Energiser Bunny) it quickly discovered it was making more money from selling pink bunnies than batteries.