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.”