Out of the world's oceans to the point of your knife

We love nosing around fish markets. At London's Billingsgate you can learn to cook the critters, too. Gerard Gilbert takes the cod by the gills on a day course

It's half past eight on a filthy morning and my hand is rooting around inside a dead squid; a decidedly chilly, deceased squid, it has to be said. We've just been shown how to extract the strange translucent quill that gives backbone to the beast, and separate the tentacles (delicious) from the eyes (not). Despite my taking great care, the creature's ink sack has exploded, leaving my hands looking like those of a messy Dickensian bank clerk. The cold rain beating on the upstairs windows of the Billingsgate fish market adds to the effect.

It's half past eight on a filthy morning and my hand is rooting around inside a dead squid; a decidedly chilly, deceased squid, it has to be said. We've just been shown how to extract the strange translucent quill that gives backbone to the beast, and separate the tentacles (delicious) from the eyes (not). Despite my taking great care, the creature's ink sack has exploded, leaving my hands looking like those of a messy Dickensian bank clerk. The cold rain beating on the upstairs windows of the Billingsgate fish market adds to the effect.

I'm here to learn about fish; how to choose it, how to prepare it and how to cook it. The Billingsgate Seafood Training School is one of a number of educational establishments mushrooming around the UK because, like a growing number of my compatriots, I am eager to acquaint myself with creatures of the sea. Indeed, it's a strange irony that almost as fast as the once-abundant fish stocks around our shores diminish, so piscatorial curiosity among the British has been growing.

The exhortations of the diet and health industry have a lot to do with this, but so does the renaissance of the British interest in cookery. Fish must be the least sympathetic ingredient to the national pastime of over-cooking everything to the texture of shoe leather, and - ergo - the better we cook, the more toothsome the fish. Either way, whereas 10 years ago baked cod on a restaurant menu would have seemed daring, now sea bream en croute merely fills us with ennui.

Mind you, this morning at Billingsgate, just like every morning, virtually all members of the public milling around the fish counters are of African or Asian or southern European origin. This suggests that, as a nation of the born-again piscivorous, we're still far from treating fish as every-day sustenance. Fish is back on the menu, but it's still an unfamiliar commodity to be treated with trepidation.

Which is why I am on this day-long course at Billingsgate, the fish market that relocated from its centuries-old home on the banks of the Thames at London Bridge to the Isle of Dogs, in the shadow of Canary Wharf, in 1982. We have been summoned to arrive at 6.30am, by which time, incredibly, the main business of the market has already been conducted. The Chief Market Inspector, Chris Leftwich, is our guide for a tour of seemingly endless stalls of fishmongers and brokers, informing us what to look for in choosing crabs, lobsters and fish. (Look the last in the eye - if a milky optic stares glassily back, walk on by.)

At this unearthly hour I'm not used to having a haddock thrust under my nose, but Chris's point is that fresh seafood doesn't give off a fishy smell. And to underline his point, the market, full to the gills with fresh produce, doesn't have one iota of that rank bins-behind-fish-restaurant odour. A little surprisingly perhaps Chris champions frozen fish. It's mostly been frozen on the trawlers, he says, and can be fresher than "fresh" fish.

After a bacon sandwich (man cannot live on fish alone) from the market café, it's upstairs to the cookery school. This consists of two large rooms - one a sort of lecture room with a state-of-the-art demonstration kitchen that wouldn't be out of place in a smart family home in Notting Hill. A mirror runs above the length of the worktop so that pupils can see what's going on inside the cooking pots. Our tutor today is C J Jackson, co-author of Leith's Fish Bible, who is busy chopping vegetables to add to a court bouillon.

Next door is a much more functional kitchen, with large, free-standing stainless-steel wash basins with the hose taps that are fashionable among the sort of people who were buying butcher's blocks a few years ago. It's here that, decked out in aprons, we begin by cleaning and preparing various shellfish: scallops, clams, mussels and prawns - the shells of the last two being added to the court bouillon to make a shellfish stock for use in the later cookery demonstration.

It's comforting to learn how to shuck an oyster without losing fingers and thumbs in the process. With the right wrist action, and protecting the other hand with a cloth, I'm soon shucking them with the ease and speed of a waiter at a Breton brasserie. But it's the cephalopods that really interest me - the squids, octopuses and cuttlefish. I enjoy nosing around fish markets in France and Spain but have always been put off buying fresh squid and cuttlefish, daunted by my ignorance of how to prepare it. Wherever you go, squid is inevitably cooked in to chewy rubber bands. The trick with squid and octopus, explains C J Jackson, is either to cook it very quickly or very slowly. As the song says, don't mess with Mr In-Between.

Anyway, we're soon making a fantastic mess - unlike some cookery courses, this encourages a very hands-on approach - packaging our neatly prepared scallop meat and octopus to take home later. Today's course is fairly under-subscribed, but most are booked up months in advance. My fellow pupils are all amateur gastronomes. The money we pay goes towards funding courses for local schools, and anyone who witnessed the canteen junk food addicts in Jamie Oliver's recent Channel 4 series, Jamie's School Dinners, will rejoice at the fact that at least some of our children have actually met a cod in the flesh. C J says kids love the whole messy, hands-on experience.

Having washed and scrubbed, we're now back next door as C J and her assistant prepare and cook a San Franciscan fisherman's stew called cioppino and octopus braised in Rioja. While those dishes are bubbling away, we're shown how to dress a crab - that is to say, how to pull one apart and smash it up so that you are left with the separated white meat and dark meat (mashed up with breadcrumbs and parsley) all laid out in the empty shell. We're to take it home to share it with our loved ones, like children showing off their schoolwork to their parents.

By now the cioppino and the braised octopus in Rioja are ready, and we eat a plateful of each, mopping up with fresh bread and sipping a crisp Loire white. You'd pay a small fortune in a London restaurant for this kind of meal. It's still only half past one as the last mouthful goes down. The day of the dedicated piscophile is as long as it is rewarding.

The Billingsgate Seafood School (020-7517 3548; www.seafoodtraining.org)