What is it about the British and fish? We are always being encouraged to eat more fish but many of us do not seem to know how to choose or prepare it.

According to Carol-Jane Jackson, the new director of Billingsgate Seafood Training School: "The British don't know how to cook fish, which is odd considering we live on an island."

Twenty years ago, you could have asked your local fishmonger for advice but many have closed down and they have became a rarity on the high street. Today, an estimated 85 per cent of the £2bn-a-year UK fish industry goes through the supermarkets, whose staff can be less than well trained. At my local supermarket last week, a sign over the fish counter urged me to eat fish twice a week - one meal of an oily variety and one of a white variety - but when I asked the assistant which of the fish on sale were which, she couldn't tell me.

Fortunately, help is at hand at Billingsgate, London's historic market for the seafood industry (the trade calls all fish "seafood"). For the past five years, the market has also functioned as a living classroom for its training school.

This is an establishment with two missions. One is to introduce schoolchildren to what Jackson's deputy calls "the fabulous world of seafood". The other is to train a new generation of fishmongers. The school's philosophy is simple but effective. The courses for children and would-be fishmongers are free of charge. In order to pay for them, the school does commercial work, for example with Waitrose (the one supermarket chain that does fully train its fish counter staff), and also offers reasonably priced classes to the public.

Jackson, or C.J. as she prefers to be called, took over as the school's director in February and its new, state-of-the-art kitchens are her pride and joy. "Previous directors came from the industry but my own background is food writing and training," she says.

A cheerful, energetic woman, she was with Leiths School of Food and Wine for 10 years, ending as vice-principal, and is the co-author of Leith's Fish Bible, among other books. "I'm here to take the school forward. Fishmongering did go through a decline but now people are starting to see there's a market in it, especially at the quality end. There's a huge amount of interest compared with 10 years ago from people wanting to start their own businesses.

"Seafood is the ultimate fast food but consumers can be put off by it. That's where a good fishmonger comes in - or you can take one of our public cooking courses and learn for yourself. It's a lot of fun."

Up to 300 people pass through the school in a busy week, from children to Conran restaurant chefs learning "knife skills". A core course is the bi-monthly "Getting Into Fishmongering" and I went along to meet some of the participants. The course leader, Charlie Caisey, is a well-known character in the business. "I've been a fishmonger for 54 years," he says. "I sold my own business 14 years ago but I still come down to Billingsgate once a week. My love is the knife and the fish. Young people thought of fishmongering as a cold, hard, wet life and turned their noses up at it but now I'm getting a lot of people who've had enough of working in offices and want a change of career.

"I blame some of the problems the trade has had on us, the older generation. We didn't really sell fish. We took it because it was there and we really did waste so much. I've thrown away squid in my time because I didn't know what to do with it. Monkfish was fed to cats. We've learnt better now."

One thing that hasn't changed over the years is the pre-dawn market hours. It was still dark when I arrived at Billingsgate to see the school in operation. The road sparkled with salt and frost like baccalà, or salt cod. The enormous car park was full of people carrying heavy boxes and trading was already winding down for the day inside the ugly, modern building - fork-lifts were shunting polystyrene boxes of unsold catch into enormous deep freezes and stalls were being hosed down.

The six aspiring fishmongers I met were smart in their white coats embroidered with "Billingsgate Seafood Training School".

Today's teacher, Barry O'Toole, one of the three market inspectors, had just taken two feisty lobsters out of their pool - a heavy black Scottish one and a lighter Canadian. He gently stroked their heads until, gradually, they became as docile as pussycats.

Trainee Liz Corny, a mortgage adviser who would like to run her own shop, already works one day a week in her local fishmonger's in Bristol for the experience. "I condensed my five-day working week into four and now I do a day for free at the fish shop. It's hard work but I believe there's a market for the more conscientious consumer who doesn't want to shop in supermarkets but at the moment has no choice."

Fellow trainee and naval architect Giles Whittaker believes "supermarket fish has probably got a few miles on it" and wants to offer an alternative. Another trainee Murat Turkoz's family is in the fish trade already "but, being Turkish, I only know Mediterranean fish. I've come here to broaden my experience."

Today's trainees hope to emulate the success of the school's star graduates, the Norris brothers. It was through Jonathan and Daniel Norris and their mentor, veteran John Wright, that I first heard about the Billingsgate School. One day last year their fish stall appeared in my local Pimlico street market, selling a quality and range of fish that made it an immediate hit. Wright used to run a high street shop until "the rent went through the roof and we lost two generations who don't eat fish - they eat beans on toast in front of the telly. Everyone in the trade will tell you the same story."

The Norris brothers used to work in information technology at the Foreign Office. Daniel, 29, says: "We were totally bored in IT and wanted to do something we enjoyed." Jonathan, 34, adds: "I could never explain what I did in IT. Now I buy something at one price and sell it at another - it's a tangible thing."

The brothers supply a restaurant chain and at weekends run the market stall, which does a roaring trade. They hope to open a shop with a restaurant attached. Jonathan admires the fish shacks and bars he has seen in Australia, which are like a cross between a fishmonger's and a fish and chip shop but with a smart, contemporary feel.

"It's quite easy to present fish well," Jonathan feels. "There's no reason why we can't do it here." He detects "a new attitude to eating fish in this country. It's just starting to happen."

Back at Billingsgate, Adam Whittle, C.J. Jackson's deputy director, says he is encouraged by the brothers' success. "Fish is one of the last hunted resources so the supply is always going to be finite," he says. "But, within that constraint, there's huge potential. The world of seafood can be quite mysterious at first, it can put people off, but with a little training, you can take the mystery out of it."

I can testify to that after joining the "Introduction to Shellfish" course (£75 including lunch and a big bag of fish to take home). For half the morning, dressed in waterproof blue aprons and plastic boots, we cleaned squid, butterflied prawns and scrubbed mussels. Then Jackson cooked chargrilled squid salad and seafood laksa for our lunch. Throughout, she kept up a flow of expert tips. I now know I should buy female lobsters (bigger body) but male crabs (bigger claws). To make sure neither suffers when chucked into boiling water, I can freeze the live lobster for a couple of hours until it is comatose. But I have to soak the crab in tap water where it will start to drown and won't feel any pain. My only disappointment? I had hoped the course would teach me an easy way to open oysters but, alas, there is no such thing.

GET HOOKED

■ Billingsgate Seafood Training School, Office 28, Billingsgate Market, Trafalgar Way, London E14 5ST, tel: +44 (0)20-7517 3548; www.seafoodtraining.org

■‘Leith’s Fish Bible’ is published by Bloomsbury at £35

■Norris Brothers Fish Stall, Tachbrook St, London SW1. Thurs, Fri, Sat