The nadir of British manufacturing was 1975, the year after the “three-day week”. British Leyland, the carmaker responsible for dreadful vehicles such as the Morris Marina and the Austin Allegro, folded, and was duly nationalised.

Meanwhile, in a makeshift workshop in a flat overlooking the Brompton Oratory in west London, a young engineer named Andrew Ritchie was focusing on folding: making a collapsible bike that didn’t collapse under the weight of the ride yet could compress into a compact size.

“Of course I was young and took no notice of what was going on in the great world around us,” he says when reminded of the industrial turbulence.

“It didn’t really impinge on our lives, we were just getting on with young people and enjoying living in London.

The Brompton bike, as it was christened, was a transportation revolution. It caught the imagination of city-dwellers and car owners because it could convert in seconds from an an easy-to-handle bundle of tubing, tyres and cables into a nimble and ergonomic city bike. Commuters, in particular, love a design that can be taken onto rush-hour trains without breaking railway rules or annoying fellow passengers. They seem undeterred by the price; the most basic model is edging towards £1,000.

The secret is an asymmetric design of elegant simplicity. The crucial step? “I somehow crossed a threshold from the ‘fold-in-half’ bikes,” says Mr Ritchie. “It wasn’t a big change, but it was enough to make the thing much smaller and that much easier to manage, so that folding bikes caught the imagination of the bike-riding public.”

Brompton Bicycle Ltd is now the UK’s largest cycle manufacturer, with five new folding bikes rolling off the production line every hour of every day from a vast new factory in Greenford, north-west London — officially opened by the Duke of Edinburgh.

Prince Philip unveiled a brass plaque that had been hand-brazed by Brompton craftsmen and women. Rather than use mass production, each bike is hand-brazed by someone who has trained at the company for 18 months.

Mr Ritchie, who accompanied the Duke on his tour of the factory floor, says of the four decades of development of the Brompton: “It was rather like having a small puppy that you pulled behind you, and it was well behaved. But it grew and grew and grew, and then it was me reluctantly being dragged forward by this large creature.

“When finally people were out there buying it and loving it, it was just really rewarding, so it was easy to keep at it.