Rigour.

In October, St John, in Clerkenwell, east London, will be 20 years old. In itself, this should not be an extraordinary thing. Two decades: it's hardly a lifetime. But restaurant years, like dog years, are different. The capital has a handful of longstanding dining institutions: bustling Sweetings, in the City; Rules, purveyor of game in Covent Garden; Wiltons, where one may eat oysters in Mayfair, assuming one has first remortgaged one's home. Pretty much everywhere else is in constant flux, joints opening and closing, chefs arriving and leaving, the crowd descending and then, ever-fickle, moving on: an exhausting and sometimes heartbreaking spin cycle of briefly modish ingredients, cuisines and cooks.

How, then, to explain St John's long, happy and singular life? Its proprietors, Fergus Henderson, formerly its chef, and Trevor Gulliver, his business partner, are damned if they know. "We just quietly go about our business," says Gulliver, coffee slopping over the side of his cup as he slaps the table. "We don't do vision." Henderson contemplatively sips his mid-morning madeira. "Yes, and there's still lots to do," he says. "There's always tweaking to be done." Better to list the things they don't believe in than those they do. Set menus? Not even when business is bad. Posh wines from around the world? France will suffice. Bawling out the staff? Not necessary. The lure of MasterChef? No, thanks. Finally, they come up with a word: rigour. "There is a rigour here," says Gulliver. "You have to be quite stubborn to do something like this."

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