The flash and crack of the arc welder pours out on to the damp cobbled street as a train thunders across the roof of the Archway Garage.

As the train passes, the little ripples in my mug of tea subside, and a huge single-cylinder engine clicks and bursts into action. Its raw thump is warm, slow and deep. You can feel the heartbeat vibrations in your chest as the engine note bounces off the workshop walls.

Old Castrol GTX and Esso sheet metal signs hang among the racks of well-used tools; a few motorcycles are raised off the oil-stained concrete floor on stands. A handful of bikes stand together, tightly packed awaiting the spanner or collection. If you had to imagine a backstreet motorcycle workshop, you would be hard pressed to set-design a more perfect scene. It has real, if almost too perfect authenticity, the ripped T-shirt oily rags, the random old bike parts on high steel shelves reaching up to the fluorescent tube-lit ceiling, the metal kettle surrounded by a selection of mismatched grimy mugs, a Norton here, a Bonneville there.

But this is no ordinary back street garage but one where nostalgia for the sixties and seventies is the newest fashion. Its home, in the coolest of cool areas of London's Camden Docks market, the spiritual home of street trend, is a throwback to lovable gangster and hard cop films, but it is as real and honest as the rain that is fighting its way through the city sky.

This is Untitled Motorcycles, one of a handful of workshops in the UK that are at the oily end of the latest automotive trend of retro customisation.

There is a style to suit almost any taste and budget. If you want highly-finished, show-ready converted seventies and eighties BMWs, Kevils Speed Shop in Devon is for you; for race-inspired bikes there's RedMax in Hampshire. But if you want street-inspired urban cool there is no better designer than Adam Kay, mechanic Rex Martin and "motorcycle beautician" Anita Chatelan in Camden. In Scotland the scene is more homemade but Clydebank's Area 51 custom shop turns out retro rocker bikes beside the US-style choppers it is better known for.

British motorcycle customisation stretches back to the 1950s, when the term cafe racer was coined. They were stripped-down machines, lighter than stock with dropped clip-on handlebars to make the riding position more aerodynamic, and were the low-cost mode of transport for the growing band of post-war rockers who would race ton-up (100mph) between transport cafes, such as the famous Ace Cafe on London's North Circular Road, along the then quiet motorway network. However, by the seventies, faster, more reliable Japanese race-inspired machines become the norm and the customisation scene was relegated to a few chopper enthusiasts. Ewan McGregor's Long Way Round created a boom for the big BMW adventure-style bikes but the image was very safe and lacked the raw edge associated with old-school motorcycling. While the city banker bonus crowd have adopted the equally corporate Harley Davidson, thinking it is a cool statement, they missed the point.The cult of the new custom is a reaction against the rush towards cultural homogeny and the lack of contact we have with increasingly high-tech machinery. It is a hankering for a simpler existence; more honest, less electronic, less corporate, in the same way that the rise of baking may be a yearning for simplicity in a world where food production is so far removed from us.

This search for authenticity is reflected in the design aesthetic. The guts of the machines are exposed. There are no fairings to protect you from the wind, no plastics to hide the machinery. The artist Conrad Leach's Lucky 13 painting - a leather-clad skeleton hurtling hell for leather - captures the mood beautifully, aesthetic and disturbing, with a look of imminent, unavoidable danger.

A custom machine these days is a much more reliable prospect than it used to be. A quick trawl through eBay's motoring auctions will soon throw up hundreds of suitable donor bikes, once loved but perfectly serviceable eighties and nineties BMWs, Hondas and Yamahas forming the bulk of the frames and engines.

Styles are also evolving. Some variants can trace their roots either to Californian beach drag races, or Steve McQueen-inspired Great Escape styling. It is your choice.

The custom crowd are adopting a clear clothing style that has its roots linked to the youth rebellion born in the rubble of post-Blitz Britain, 1950s rationing and 60s conformity.

But unlike the days of the "ton-up" boys who had to hunt for second-hand flying jackets and make do with street jeans and poor safety kit, the new crowd has its own set of bespoke and rekindled heritage manufacturers who are making stylish armoured jeans, jackets and helmets. Notable are Galashiels' Aero Leather Clothing and London's Lewis Leathers.

There is always a danger that, as with any trend, as it grows it becomes mainstream and absorbed by the big commercial brands. This is Harley's problem - what once was cool becomes adopted by the crowd and it loses the very people who made it cool.

But for this custom trend there is hope. There are probably 100 different kinds of old bike that can be customised and an endless variation of lights, tyres, colours.

It also requires you, the individual, to research and make your own choices. So for me, it is a white frame, 3/4 tan seat, seven-inch headlight, film tub flashers, blue/grey matt tank, and a 23-year-old stripped-down Dakar Desert-inspired Suzuki enduro bike.

Cover your ears, the Big 8 is here. n