During the closing week of the trial BBC Radio 4 broadcast three extracts from The Holy Blood and the Holy Grail. Listening to these readings, it is easy to see that Dan Brown drew heavily on the work that the Holy Grail is not a vessel, or rather is a vessel, but a very special vessel, The Mary Magdalene, that she married Jesus, that she escaped from the Holy Land with her children, that these formed a Holy Blood Line in France, this can be interpreted from how one divides the Old French words San gréal or Sang réal, the first being Holy Grail, the latter Royal Blood, the Priory of Sion being an inner circle inside the Knights Templar established to protect this bloodline, famous names being Grand Masters, hidden wealth in an obscure part of France, links with churches dedicated to Mary Magdalene and so on.
On top of this, Dan Brown weaves his magic as a story teller, the modern day Grand Masters, the hunt by our heroes, the clues left by Leonardo Da Vinci and so on.
Curiously, Dan Brown denies being influenced by The Holy Blood and the Holy Grail, having only given it a cursory glance, then having put it down as impossible to read.
There is is no mention in The Da Vinci Code of the 'mystery of Rennes-le-Chateau', the first extract broadcast on BBC Radio 4, although curiously, why use the surname Sauniere? There were exchanges in October about why Mr Brown chose the surname Sauniere and why he chose the surname Teabing.
It is self evident that Mr Brown looked at The Holy Blood and the Holy Grail before The Da Vinci Code was finished - he accepts that.
It is equally self evident that Blythe Brown looked at it extensively.
The original copy as disclosed contains numerous annotations and markings mostly by her, but also by Mr Brown. Of all the books used it is the most heavily annotated.
I have already observed the anagram in the name Teabing as being another example of how The Holy Blood and The Holy Grail was clearly in the mind of Mr Brown when he finalised his book.