DAN BROWN got a date wrong in The Da Vinci Code. The error may well prove to his advantage.

According to him the Priory of Sion, alleged keeper of the secret of Christ’s wife and children, was founded in Jerusalem during the Second Crusade in the reign of Baldwin II. But according to the authors of The Holy Blood and The Holy Grail, who are suing Brown for stealing their plot, the Priory was founded in 1099 during the First Crusade, and Baldwin did not ascend the throne of the ancient city until 1118.

Rarely has medieval Christian history had such a field day in court, but then Mr Justice Smith, who is hearing the Chancery Division case alleging infringement of copyright, is equal to the task.

Having done his homework, he knows that the Second Crusade lasted from 1147 to 1149.

In his third day in the witness box Michael Baigent, co-author of The Holy Blood and The Holy Grail, was being cross-examined on his claim that Mr Brown had lifted his book wholesale. John Baldwin, QC, for the defendant, Random House, which published The Da Vinci Code, suggested that Mr Brown had got his information from an entirely different book, a history of the Knights Templar.

The judge turned to Mr Baigent: “Well, he certainly didn’t get it from you, because you wouldn’t have made the date error that he has.” Mr Baigent has been having a tough time at the hands of his inquisitor, and has been forced to concede that numerous details of his case are wrong. But there are moments when he fights back gamely, and is even conciliatory towards Mr Brown, who sits silently absorbing every word and awaiting his turn in the witness box next week.

“I don’t hold it against Mr Brown, as he is not writing a work of history,” Mr Baigent said of the thriller writer’s getting the wrong Crusade. “But I don’t feel it’s up to me to correct his creative use of history.”

Mr Baigent was beaten into retreat on his assertion — a vital plank of his case — that all the historical conjectures in The Da Vinci Code were lifted from The Holy Blood and The Holy Grail. “I concede that ‘all’ is far too strong, and that we should have said ‘most’; I confess we over-egged that one,” he said after Mr Baldwin had pointed out that Jesus having children, Lazarus being his brother-in-law and Mary Magdalene and Mary Bethany being the same person, all postulated in The Holy Blood and The Holy Grail, did not figure in The Da Vinci Code.

“Does it not trouble you that you claim Mr Brown reached all the same historical conjectures when he plainly didn’t?” Mr Baldwin asked. Mr Baigent refused to take that entirely lying down. “Mr Brown is writing a novel; he doesn’t need to go through all the arguments. We agree that Mr Brown uses the marriage of Jesus in DVC; that is a development of our researches.”

Another major theme of The Holy Blood and The Holy Grail is that the Holy Grail is not a chalice but the bloodline of Christ and his descendants. “None of the situations you develop in HBHG in connection with the Grail romances is in DVC, is it?” Mr Baldwin challenged.

“But our conclusion [about Jesus’s bloodlines being continued through the early Merovingian kings of France] forms an integral part of DVC. The novel is using the fruits of our conjecture,” Mr Baigent said.

The judge, who adjourned the hearing for six days so that he could absorb both books, is sitting without a jury. He told the claimants yesterday that the part of their case that relied on similarities of language between the books was “extremely peripheral”, and that “casting a Nelsonian eye” over selected passages was unlikely to get them very far.

Sales of The Holy Blood and The Holy Grail, first published in 1982, have increased sevenfold since the hearing began last week — 3,000 copies — and a few days ago British sales of The Da Vinci Code passed 4 million to add to worldwide sales of 40 million.

The hearing continues.

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