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These are momentous days for the Brompton, now easily Britain’s bestselling and most exported bike.

The present design is celebrating its 25th anniversary — and over the weekend the 10th World Brompton Championship was staged over a 15-kilometre course around St James’s Park and The Mall.

First prize for the three races (men, women and veterans) was a sleek, limited edition Black Brompton.

A special prize went to the best dressed competitor — with a warning note: strictly no Lycra.

This year the little Brompton factory under the M4 in Brentford will produce 50,000 bicycles, 80 per cent for export. In Tokyo and Bangkok they have become fashion items.

Plans for a bigger factory are to be announced this week. It will be in London and have the space to turn out 100,000 bikes a year. An electrically assisted model is expected next year.

Brompton's sleek new Black Edition bikes 10 show all Brompton's sleek new Black Edition bikes 1/10 Berry Crush & Black Brompton 2/10 Black & Black Brompton 3/10 Lagoon Blue & Black Brompton 4/10 Lime Green & Black Brompton 5/10 White & Black Brompton 6/10 Berry Crush & Black Brompton Folded 7/10 Black & Black Brompton Folded 8/10 Lagoon Blue & Black Brompton Folded 9/10 Lime Green & Black Brompton Folded 10/10 White & Black Brompton Folded 1/10 Berry Crush & Black Brompton 2/10 Black & Black Brompton 3/10 Lagoon Blue & Black Brompton 4/10 Lime Green & Black Brompton 5/10 White & Black Brompton 6/10 Berry Crush & Black Brompton Folded 7/10 Black & Black Brompton Folded 8/10 Lagoon Blue & Black Brompton Folded 9/10 Lime Green & Black Brompton Folded 10/10 White & Black Brompton Folded

This is a success story that nearly didn’t happen. Engineer Andrew Ritchie made folding bike carts for his gardening business before building his first folding bicycle in a bedroom overlooking the Brompton Oratory in 1975. Within three years he had perfected the design.

He asked several famous bike names to help make and sell the bike. They all said it wouldn’t work, so he went ahead on his own, and was proved right despite one or two bumps in the road.

The secret of the Brompton’s success strikes you on entering the factory. Each bike is hand-crafted, with the initials of the main builder at the base of every frame. The bicycle’s strength lies in the quality of steel and the way it is bent, treated and joined together in a process called brazing.

“The two pieces of steel to be joined are heated at 800 degrees centigrade,” says Abdul el Said, 54, who is in charge of training. “The bonding brass has to be heated at 1,000 degrees. Every join is distinctive — I can tell you who of the 44 brazers has made each one. It’s like jewellery, very individual.”

Training in brazing can take 18 months. Rebecca Summers, 24, who is several months in, said: “I was working somewhere else in the factory and was fascinated by brazing so asked if I could apply. They said yes. It’s wonderful — I hope to be here for years to come.”

I asked Abdul, who worked for a German-Italian cycle company in his native Lebanon, why the Brompton works. “Design, sheer simple design — it’s the best I have ever seen in the business,” he said. “That, plus the made-in-London label, gives you the full formula for success.”

Over the next 10 weeks, the company’s website will give details of a competition to design your own Brompton, from all the different brakes, gears, handlebars, lights, mudguards and so on. “It’s not as easy as you think, there are 16.5 million possible combinations,” says Nick Charlier, the new external relations manager. The winner gets the bike they have designed.

The best designs so far were on show at the World Brompton Championship at the weekend. There was also a contest to fold and unfold a bike fast enough to beat the record time of 5.6 seconds.