Not much gets made in London nowadays, so it comes as a surprise that there's a proper factory in Edgware, a few miles north west of Hyde Park. Inside, there's an output target scrawled in chalk on a blackboard declaring that this year Boosey and Hawkes aim to turn out £9,544,055-worth of musical instruments here.

It hangs alongside newspaper clippings telling how the future of the works, and the £7.7 million-profit business of which it is part, is in doubt, and not because of failure either. A big American stakeholder wants to pull out because of its own impending sale, and no-one knows whether the legal requirement that the shareholding must go to a single new owner can be fulfilled. The music business appears not to be flush with entrepreneurs who fancy continuing B&H's tradition of producing both the software (sheet music) and the hardware (instruments) of music-making.

B&H owns companies across Europe and the States which between them could equip an entire orchestra, bar the piano. The Edgware works contributes most of the brass section, especially for brass bands, and flutes. It's not grandiloquent like the nearby Hoover factory which was converted into a Tesco's. But every Wednesday, visitors troop round it to see 350 real people manufacturing real things. They are lucky. No concessions are made. There is a decent grime about the place. The few smokers, smoke. There are pin-ups - though they are fairly polite. This is not a theme park.

The works is not especially distinquished, though it was purpose built by Hawkes in the 20's to make instruments. Its between-the-wars red brick and zig-zag roof sits well with the parades of shops (many replete with erstwhile picture palaces) which line the Edgware Road's straggle into London.

The suburbs round here are strikingly Asian, so in some departments - especially flute-assembly - there are rows of women in saris, Hindus mostly, from all over the East. Shanta Peshavara, for instance, has worked here "Eighteen years - too long!". But she's laughing, and proud of the fact that whilst her parents were labourers in Uganda, she has architect and businessman sons here. She and her friend Hansa Katwa (20 years at Boosey and Hawkes, and before that Tanzania) were doing final assembly work on some tenor horns. The sinister acid baths upstairs had inadvertantly put silver-plating into the screw-caps of the valves. The instruments, which will probably spend their lives playing marches from the age of Jingo or show tunes from Broadway's heyday, were reground to perfection to the subdued tones of Sunrise Radio's Bollywood tunes, out of Southall.

Wherever you walk, there are raw materials: tube and sheet metal, mostly. As the intruments come together, you begin to spot sections, variously bent and flared, lying about like limbs waiting for bodies to join up with. On some benches, there are boxes of valve-assemblies, or they hang - like guts on a butcher's hook - above the assemblers' heads.

The flared ends - the "bells" - of sousaphones look like traffic cones, at least until they are joined to the rest of the instrument and finally shaped into something more sinuous. Then they begin to take on something of the form of an old-fashioned ship's air vent.

The visitors get to see craftsmanship as though in a time capsule buried somewhere about the time the factory was built. Here a man is bashing away at a sheet of brass with a wooden mallet, with a suitably deafening racket. There, there's a forge which would be perfectly in place in a blacksmith's or a farrier's. In one spot, the whole building takes up a sympathetic resonance: it hums and vibrates with the power of the machines around. A man shows you how to put a perfect bend into a tube whose inside must be unkinked, as round as possible: first put a long dent into it, then bend it, and then push out the dent with an ancient-looking water-press which blows hundreds of pounds of pressure. Elsewhere, an eight foot band of polishing ribbon flies round and round, whilst a man in a mask (well, most of them bothered) proffers up a bit of an instrument for industrial-scale buffing.

At the other end of the scale from these gutsy processes, Mark Parfitt does the fine soldering which first makes valve assemblies and then builds them into complete instruments. "I came here a couple of years ago as a musician to test the instruments. But I play them at night as well, in the Regent Brass Band at Wembley, and it all seemed a bit much. So I thought I'd try my hand at assembly." He has a pencil of blue flame at his elbow, and the solder sizzles onto the tubes he's heated up ready.

Polished and repolished, the instruments go for engraving - a computer driven bit etches out the filligree'd company initials, and the BESSON brand name, classic and refined, and the instrument's serial number. So the one bit of the process one would have sworn has to be hand-done, isn't. Keith Mason in design has the only other computer. He has a different Winnie the Pooh tie for every working day and used to play electric bass in a four piece dance band until Scouting and families ties called for retrenchement. "We do reverse engineering here", he says. Once a new instrument shape is found to work, its proportions are reduced to numbers by the machine. Patiently, he takes an innumerate through the mechanics which underly music-making.

At the other end of the production process, Tom Eveson - a Regent player, too - tests every instrument. First, it's given a sort of blood test: the bell is blocked off and air pumped in. The valve seats and cushions, the welds, and the sliding tubes, must all be well enough sealed to shift the liquid in the pressure gauge.

And then cosmetics. Eveson is finnickety to a degree: masking tape marks the offending patch of poor plating, or a weld which is messy, on a euphonium (B&H's are said to be amongst the best. along with their tubas). It ends up like a very shiny baby littered with sticking plaster. "This one's not typical", he apologies. Only then is the instrument fit to be heard. Against a background of Capital Radio, Tom Eveson - like a professional deflowerer - takes the virgin and blows a note through it. In case his ear isn't enough, he also blows the instrument towards a stroboscope which proves the instrument is pushing out the right frequency. The magic in the plumbing, the poetry in what might otherwise be highly-polished exhaust manifolds, becomes clear. The rest is up to human lip, ear and brain.