Tim and Nina Zagat don't look like people who changed the way we eat out. They look like the sixtysomething, affluent visitors from New York that they are. But with their Zagat guides, launched in New York 30 years ago next month and in London in 1997, they democratised restaurant reviewing by simply asking people to report on where they liked, why they liked it, and publishing it.

“Thirty years ago, London was not a very good place to eat,” says Tim Zagat. “Now, with New York and Tokyo, it's one of the two or three best places to eat in the world.” He believes that's partly because of the sheer variety of food available in the city, and also because lifestyle changes - men and women working longer hours and having less time - mean that more people want to eat out more often. And because London has restaurants in every price range, and the state of the pound is making it more attractive to tourists, restaurants in the city are weathering the credit crunch well.

“People are not tightening their belts that much, but they are being a little more careful in choosing where to eat. And they are not showing off with the £200 bottle of wine any more. People want choice: they want a French meal today, an English meal tomorrow and a Thai meal on Thursday.”

But what they are also increasingly demanding is what the Zagats call greening - food that is local, sustainable and organic. And following on from the smoking ban, they think the transfat ban should also cross the Atlantic “because the doctors think it's not a good thing to be consuming”. There are plenty of people who sneer at guides such as Harden's and Zagat, which rely on the reviews of diners not professionals. Who cares what the unwashed masses think, they say. Of what value is it to have scores for decor, cost, service and food and a three-line review that reduces Gordon Ramsay's flagship restaurant to [their quotation marks] “crisp, chic” and for “serious foodies”? Over breakfast at the Wolseley (sausages, scrambled eggs) Tim is having none of it.

“A thousand people saying they like the sausages here has more meaning than one critic. Our guides try to make it easy for you to make a quick decision, reliably. We cover something like 1,200 restaurants in London. It would take a critic years to do that.” The two can and do co-exist comfortably: a critic, he argues, paints a full-colour portrait of a restaurant “and hopefully gives you a good, interesting read. We're trying to help you to make a quick decision when you want lunch or to grab something light before a movie.” They now have 350,000 reviewers worldwide, and around 5,300 in London. The books sell in their millions, from Los Angeles to London and Sydney to Singapore. Yet when the couple, who met at law school and were then practising lawyers, first proposed the idea to publishers, every single one turned them down.

“They all said ‘Why would people want to know what other people think?'” says Nina. She now takes great pleasure in seeing how much people rely on their guides, which have expanded to cover hotels, spas, golf courses and nightlife, and in hearing how they've tried new restaurants because of Zagat's.

So has the London restaurant scene overtaken New York? Is it the best in the world? “No,” says Tim. “But it's very close.” He thinks Paris was the clear winner 20 years ago, and is still the best place for French food, but lacks the diversity that makes London and New York stand out.

“The French love French food so much that they haven't been quick to move away from it. In London and New York, maybe because our own cuisines were not so robust, we were much quicker to accept a lot of other types of food. And both cities are very international.” As they don't contribute their own opinions to the guides, they are reluctant to name their favourite restaurants. But in London, Tim admits that he rates Hakkasan “because I think it has better Chinese food than any place in New York”. They like instead to pop in to as many as 20 restaurants a night, not to eat, just to see how busy they are, what the atmosphere's like, who's there, and to check out the menu.

“I like to get a feel for what's going on,” says Tim. That, in a nutshell, is why people buy Zagat's.

The 2009 Zagat London Restaurants Guide is out now. www.zagat.com

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