Billingsgate trades up to 200 species of fish a day, but how to tell which salmon, sole or shellfish is the best to buy? Annie Bell rises at dawn to get an expert view. Photographs by Dario Mitidieri
The number one essential for becoming a fishmonger seems to be an appreciation of early mornings. Very early mornings. A dark, sleepy, silent start. It's far too early for breakfast, and at that hour most radio programmes appear designed to help insomniacs back to sleep.

But, when the weather's good, to drive through London's deserted streets to Billingsgate at 4.30am is to see the city at its most beautiful - calm, empty spaces, and serene buildings that normally fade into the confusion of people and cars.

Even unprepossessing Billingsgate looks stunning at this hour: a shallow, red-brick block, set dusky pink against a luminous blue sky.

Billingsgate is the oldest of London's four major markets, older than Smithfield, Spitalfields or Leadenhall. All manner of commodities, including corn, wine, coal and salt, were once traded there until, in the 16th century, it was given over exclusively to fish.

The old site on Lower Thames Street relocated to Docklands in 1982 and now spreads itself over 13 acres - though you wouldn't guess it from trundling past on the Docklands Light Railway. Into this ergonomic block are crammed 98 stands, 30 shops, storage units and a couple of cafes.

And a shellfish boiling room - a long, thin room shrouded in faint mist, with a row of steel vats of boiling water covered with brown scum that belch great palls of steam. Wide-meshed baskets, some four feet in diameter, full of cockles, whelks, winkles and crabs, are hauled up on rope pulleys, then lowered into the seething vats.

The market is "open" - that is, the likes of you and me have the right to shop there - but most buyers are a purposeful set from fish and chip shops, fishmongers', fish suppliers, restaurants, pubs and hotels. Traders will have been going about their business since 2am. By 7am, the show's over.

After 20 years of trading as a fishmonger, Philip Diamond, of Covent Garden Fishmongers in Chiswick, is programmed to the early-bird routine, four mornings a week, even if I'm not.

He hasn't always been a fishmonger; life before wet fish took hold found him as a taxi driver. You'd have noticed him then, as well: 16 stone of Romanian with a bushy, salt-and-pepper beard, a studied gruffness and a big, booming laugh. "People down Billingsgate think I'm hard," he grins, "I'm not, it's an act. Market's where I do the orders, shout and scream, and perform a little bit." Up and on the ball, and he still doesn't get home until 7pm... in bed by 9pm.

Now, as every fish lover knows, fishmongers are a rare breed, and there is a scarcity of good shops. Those that exist have a catchment area far broader than their immediate vicinity. Steve Hatt, a fishmonger in north London, says that, on a Saturday, some customers will drive for several hours to stock up on fish, some probably for freezing when they get home.

Why should anyone go to this trouble, given that there must be a supermarket closer to hand? Lovers of good fish know that fish are an enigma of different shapes, with skin and bones in different places, and that preparing them for the pot is no child's play. A good fishmonger is happy to employ his skills in buying, cleaning and filleting, and freshness is guaranteed.

Then there is advice. Someone who has opted for a career in fish and has a pride in the product wants you to enjoy your meal. Advice handed out on cooking and preparing can be invaluable, especially when you are preparing a given fish for the first time.

For Phil Diamond and other good fishmongers, the fish procured at Billingsgate is only a slice of the whole. The greater part of their buying takes place at quayside auctions on the coast, where an agent will buy on their behalf.

Competition can be stiff: it is said that price is governed by the size of the road; if the Spanish can get a lorry in, the price shoots up. And if it's fish such as turbot, brill or monkfish, and happens to be in short supply, the Continentals will pursue it with a passion, "like bidding against Liz Taylor for a diamond," says Steve Hatt. And they pay cash, which means everyone else has to follow suit.

But there's no guarantee of what the boats will bring in, and anything exotic or imported has to come via the market. A trip to Billingsgate is a way of keeping up with what's going on.

On arrival at the market you come across a parking attendant, the first of a hierarchy of men in white coats, and leave your vehicle in his lot - ours is called Black Jim. There are ten in all; others' names include Skinner and Bunny. Who's Black Jim? "Dunno," the attendant shrugs. "My father didn't know, and my grandfather didn't know."

Porters with barrows buzz around like worker bees. The traders will have between one and four porters in their employment who collect sales orders and barrow them out to the buyers' vehicles. For every order, they receive "bobbin money", a small percentage of the order.

The porters are dressed in white sailcloth smocks and white hats; they also have flat-topped leather hats which come out on special occasions. Sensible shoes are Wellington boots. It's wet and very slippery. After years of practice, you learn to go with the skids. For the novice, it's an ice rink.

Crushed ice fills every crevice and corner, and there's constant flushing out and washing down with hoses. White polystyrene boxes that squeak in friction display samples of the fish for sale: sleek wild salmon; lobsters with their claws taped, stacked in boxes according to size; a 14lb pike; ochre-tinged carp. A tall, steel cabinet reveals a mass of writhing eels.

Whatever you're after, the chances are you can find it. The trick is not to get done in the process. It would be pleasant to paint a picture of Billingsgate as a wondrous market where all the fish have clear eyes and perfect gut cavities, but there's substandard stuff knocking around there, too. This said, all fish coming into market is checked by "fishmeters" - inspectors appointed by the Fishmongers' Company, who are allowed to seize any they do not consider "fit for man's body". There is also routine sampling of shellfish.

For the likes of Phil Diamond, who has been going there for 20 years, the traders are his friends - but they're still going to stitch him up if they can. "If you want rubbish, you can buy rubbish, and you'll pay big money for it."

Chris Newnes, who offers an impressive range of imported fish, opens up a box with two immaculate bluefin tuna inside. "How much?" asks Phil. "Had difficulty getting these. pounds 4.80," Newnes replies, quoting the price per pound for a stone of fish. "Oh, come on, don't be stupid. I'd like it for a lot less than that." "Yeah, well, what you'd like and what you'll get are different." No deal; we move on.

"Problem with you, Phil, is you've got no respect," Newnes shouts after us. Next stop, beautiful wild salmon. Diamond's offer is met with: "Getta outta here." Later on, we go back. He may have had a bad morning, Diamond explains; we'll have another go. Diamond catches his eye, and he shakes his head. No deal. He's had a good morning.

For Phil Diamond, the beautiful and ugly are blatant to see. "Use your eyes, use your nose." He inspects the fish, feels how firm the flesh is, looks inside the cavity - is it dull, is it bright, is there a healthy coating of slime? He can tell how long it sat in the cold box on the trawler. He's looking for quality, and that at the right price.

There aren't many women in Billingsgate. We stop and talk to one of the few, Liz Holmes of CT Holmes, who inherited her father's line in frozen prawns. She has a two-year-old daughter who doesn't see her from Tuesday to Saturday. My presence is greeted with a gamut of banter, ranging from "new daughter, Phil?", to "new granddaughter, Phil?", depending on the level of impertinence intended.

By the time we leave, the startling blue of the morning has faded to silver. Driving away, we have a sense of emerging from some massive source of energy. The rush hour along the Embankment, by comparison, seems sluggish.

Back at Diamond's shop, the fish is unloaded and laid on a bed of crushed ice. For Covent Garden Fishmongers, it's a classy display of halibut and scallops, sea bass and pomfret. You can't shift herrings, sprats or cockles in Chiswick. But go elsewhere - Brixton, for example - and there'll be boxes of more modest offerings stacked up. What's on offer isn't so much the choice of the fishmonger, as that of his customers.

When it comes to cost, then the professional approach is to talk about the net meat value - how much fish is left after it has been filleted or prepared ready for eating. A fish that sells for pounds 4 per pound might only yield a third of its weight, whereas something like bluefin tuna will sell for around pounds 7 per pound and there will be no wastage.

On a sociological level, it is hard to explain why our French, Spanish or Portuguese neighbours are willing to pay prices for fish we would balk at - one reason why so much good quality stock finds its way abroad. We should value fish more: it is an obvious alternative to meat. And, as Philip Diamond says: "the harvest of the seas is hard won"

Covent Garden Fishmongers, 37 Turnham Green Terrace, W4 (0181-995 9273)

Steve Hatt, 88 Essex Road, N1 (0171-226 3963)

Next week: Annie Bell begins a series on how to choose and cook fish Fish takeaway: porters in white sailcloth smocks and white hats barrow the order out to the buyer's vehicle for 'bobbin money'

Fishy business: lowering the baskets of cockles, winkles, whelks and crabs into vats in the shellfish boiling room (main picture). Two prime flatfish (below) from the 25-30,000 tonnes of wet fish sold a year

Waiting for a bite: Billingsgate's 98 stands and 30 shops (below) are crammed into a 13-acre Docklands site. Traders (below left) sell to fish and chip shops, fishmongers, wholesalers, restaurants and pubs, as well as the occasional private customer