Da Vinci Code vs HBHG

I have heard about the court case in England where the publisher (or is it the author) of the 70's book "Holy Blood, Holy Grail" are suing the publisher (or author) of "The Da Vinci Code" for copying their ideas. Authors are following this case with interest, because it might define what it means to copy an idea. I have read neither book, but I believe that HBHG has the form of an academic study, whereas TDVC has the form of a fictional novel based on the same ideas as HBHG. Since neither are based on fact, is it legal to copy the ideas.
All this seems to be a messy battle over profits and far removed from the great ideas both books claim to be true. Truly, God is not mocked.

Is the following a fair comparison....
Thomas Keneally wrote "Schindler's Ark" and said he studied the story, spoke to as many witnesses as possible and made his novel as faithful to fact as possible. If the story had been documented in a book, would it be plagiarism to then base a book on it? Maybe that fact that the story is true (unlike HBHG) is a defence.