One further point to add to this:

Both their books are essentially baloney — made-up versions of the
past, mixing occasional facts with bare-faced invention, apparently
passing themselves off as some form of hidden truth. Their success owes
less to the strength of their narrative or the richness of their prose
than to the widespread belief, which they have done little to
undermine, that they have disinterred a secret history, covered up by
the Church for hundreds of years.

This makes the case, therefore, as much a test of history as
it is of plagiarism. You cannot plagiarise a genuine historical fact,
only a made-up one. If I can demonstrate that my Faust is based on hard
and proven research,that the man existed and that his case is
meticulously documented in the 16th-century archives of Wittenberg,
then I can genuinely claim that I am recycling history rather than
stealing a good story. I will be able, with the greatest confidence, to
fight off the heirs and successors of J. W. Goethe, should they care to
challenge me.

This then must be the dilemma for those who seek to defend
their own versions of made-up history: they can only sue and win if
they can demonstrate that their ideas are fictional, invented and
therefore entirely their own property. If, on the other hand, they
succeed in proving that their research is genuine, and their
investigations firmly based, then they have made a signal contribution
to history — a history that belongs to us all. Their revelations become
as public as the secrets of the Enigma Code, the Suez pact, or the
British presidency of the European Union.

The great joy of this case is that Dan Brown is indeed stating that it is all fiction. But the authors of The Holy Blood and the Holy Grail were, when their book was originally published, stating that it was indeed true history. So, either they were lying then or they’re lying now, when they say it is fiction which must be protected from plagiarism.

What fun, eh?