A KEY piece of the action in The Da Vinci Code takes place in the Temple Church, off Fleet Street in London. The Temple was built in the 12th century by the Knights Templar, an order of crusading monks founded to protect pilgrims. It has two parts: the round church - consecrated in 1185 and designed to recall the circular Church of the Holy Sepulchre in Jerusalem - and the chancel, built in 1240 by Henry III, who intended to be buried there. The round church is famous for the nine life-size stone effigies of knights, shown for ever poised to leap into action at the second coming.

In The Da Vinci Code, Langdon, Neveu, and Teabing arrive at the church in the belief that they are on the point of solving a riddle connected with the Knights Templar.

As the novel says, the Temple Church is off the beaten track. "Few people even know it's there," says one of Brown's characters. The Temple's own website describes it as "until recently, one of London's best-kept secrets". The runaway success of the novel has put paid to that.

"We've seen a vast increase in visitors, despite being tucked away and easy to miss," says the Revd Robin Griffith-Jones, Master of the Temple. "Numbers were up fivefold during the tourist season. People are now taking the trouble to find us."

We met soon after filming for the movie had finished. The experience, he says, was very positive. "There were well over 100 people here on the day, but they recognised that the fabric was precious and this was a special place, and were respectful throughout." To turn the church into a crypt, the crew had blacked out the windows and pumped in smoke. They even built two additional effigies of medieval knights, which he says were so convincing they almost fooled him.

For the past year or more, Mr Griffith-Jones has given a weekly talk every Friday lunchtime, under the title "The Da Vinci Code: Fact and fiction" (cost £3). He confesses he was rather slow off the mark. "It was well over a year before we twigged, and another six months before we did anything about it."

Now, though, up to 200 people from all over the world attend each week, and his talk has evolved into a book: "The Da Vinci Code" and the Secrets of the Temple, published by SCM-Canterbury Press ).

THE DAY I attend, numbers are lower (it is a dull January day), but 70 or 80 visitors still cluster in the chancel. The age range is wide, but the audience is predominantly young. The Master warms us up by denying any connection with the fictional Master of the Temple, who is famous for his bad temper, and claiming he unsuccessfully tried to wangle a walk-on part in the movie. He asks if there is anyone in the audience who hasn't read the book (surprisingly many, it emerges); and he praises Dan Brown's achievement in writing such a page-turner that 40 million people have gone out and bought it.

Then he puts the knife in: "There are occasions in the novel when Brown comes very close to saying something important and true," he says; but at every instance the novelist "veers off into outer space". Factually, there are huge mistakes on almost every page.

Mr Griffith-Jones sets out Brown's version of Christian history, and then point by point, he puts us straight. The novel is wrong about the Priory of Sion, wrong about Opus Dei, wrong about the Knights Templar, wrong about Leonardo's Last Supper (he passes round two reproductions to illustrate his argument), and wrong, too, about the relationship between Jesus and Mary Magdalene.

The talk is, by turns, learned and entertaining. At the end, he invites us all to stay and look round the Round Church. Most people linger to look at the ancient effigies, which are beautiful and moving. Many more ask questions or follow up points he has raised in the talk. Bob, a lawyer in his 50s from Washington, who is in London on business, says he found the talk fascinating, though he hasn't read the book. "I'm Jewish, but my Catholic friends told their priest: 'You'd better go read it because everyone is going to be talking about it, and you need to be prepared.'"

Elin, 26, a postgraduate student from Norway, says that the book has been controversial in Norway, too. "One reason I came was to see if anyone would stand up and object," she says. She seems faintly disappointed by the apparent good will of the audience. Guido, a young Italian in biking leathers, starts talking intensely to another visitor. He's convinced there is more to The Da Vinci Code than meets the eye. Everyone is talking about it in Italy. As for himself, he thinks it significant that he has been drawn to the Temple on Friday 13th.

Mr Griffith-Jones says that the day's crowd was fairly typical. He welcomes the opportunity to discuss the book with visitors. "I'm constantly fascinated by the questions people ask, and the fact that they are asking them," he says. " The book has given people the chance to ask questions which are, after all, very important."

It's a generalisation, he says, but he tends to find that many  Americans who attend the talk believe that Dan Brown has said something very important. British visitors, on the other hand, want to be reassured that Brown' s picture of the world is not true. "Every week there is someone who thinks you are doing Dan Brown an injustice; but, on the whole, most people seem to find the talk heartening."

On the map: above: visit Britain's map of Da Vinci tourist sites (labelled with stars). Visit Britain is using the opportunity to recommend other important historic attractions to visitors (marked with numbered squares).  On the map: above: visit Britain's map of Da Vinci tourist sites (labelled with stars). Visit Britain is using the opportunity to recommend other important historic attractions to visitors (marked with numbered squares). www.visitdavinci.com below: the Louvre has seen record numbers of visitors

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