Sybil Kapoor meets Billy the Fish, aka William Black
Standing in the mud, looking at the wrought-iron skeleton of Fish!, William Black explains how he is part of a vanguard movement to reintroduce good fresh fish into our lives. Trains thunder across the near-by viaducts, while lost-looking tourists peer at us from the safety of Southwark Cathedral. According to Black, this building site will soon become a prototype for a 21st-century fishmonger. He fully intends to break the hold that supermarkets have over fish shoppers, and I suspect that he may succeed.

As the cold air starts to bite, we head for the steamy pleasures of Borough Cafe, one of his favourite haunts. As soon as the mugs of milky coffee and piles of hot buttered toast are on the table, Black unbuttons his heavy waterproof (and, no doubt, fish-proof) coat and relaxes a little. "I love the smell of fish," he confides with a shy smile. "I really miss it when I am not handling any, so much so that I even dream about it."

His history is a strange one. After studying anthropology at Sussex University, he did various jobs in London, including selling Irish oysters from a van. "I'd never seen an oyster before, and I was the only member in my family that wouldn't eat fish," he recalls wryly. "But I have a slightly obsessive character, so I soon wanted to know everything about them."

One of his customers, Pierre Koffman, chef/patron of La Tante Claire, encouraged him to branch out into selling other fish, and before long he had moved to France to supply the English and French markets. Then love intervened in the form of Sophie Grigson. Marriage, A television series (Travels a la Carte) and fish consultancies followed, in particular with the Marine Stewardship Council - an unlikely collaboration between the Worldwide Fund for Nature and Unilever which is introducing the concept of certifying fish from well-managed or sustainable sources. "One of the most important aspects of Fish! is that we are going to try and only sell fish from these sources," explains William Black, "so that people know what they are buying."

In essence Fish! is a 90-cover restaurant with its own shop. "It's the only way you can make a small fishmonger pay these days," he says. Computers will flash up information on the choice fish of the day with some culinary advice. Sashimi-grade tuna, almost impossible to buy from a fishmonger, will be sold alongside smoked fish, bacalhau and line- caught fish. City customers can even order fish for their supper, pay by credit card and collect it from a special trolley at London Bridge Station on their way home.

The concept behind the restaurant is simplicity itself. Aside from regular items, such as a swordfish club sandwich or grilled lobster and chips, there will be a main list of about 21 fish, although only a selection will be available at any one time, depending on what is best on the day. The customer can then choose between having it grilled or steamed and served with salsa, Hollandaise, herb butter, relish or olive oil. As we talk, William suddenly remembers whitebait and scribbles it on his hand. "You have to serve whitebait in a restaurant near the Thames," he exclaims, leaping swiftly from the romantic image of 19th-century whitebait feasts to the legal technicalities of catching mixed small fry. So who else is behind such an operation? The men from Cutty's, Tony Allan and Ron Truss, the two fishmongers who opened London restaurant Bank with Christian Delteil. Years before, together with Black, they had speculated about whether it might be feasible to create a fish restaurant and shop. When William saw the current site, he knew its time had come. If all goes well, it will be the first of many

Fish! opens in the second week of February at Cathedral Street, Borough Market, London SE1 (0171-836 3236). Monday-Friday 11.30am-3pm, 5.30-11pm. Saturday 5.30-11pm