Baigent and Leigh's case, however, seems totally specious. One can protect the actual text of a piece of historical scholarship. One can, up to a point, protect the argument. If I were to write an article with new analyses and conclusions, and someone then wrote an article that followed the same train of thought, but used different words, that might be a breach of copyright.
What Baigent and Leigh are arguing, however, is that Brown appropriated the 'fruits' of their research. This, it seems to me, is not protected under law, because the fruits of Baigent and Leigh's work is what they claim actually happened, and you can't copyright historical fact. If they admitted that The Holy Blood and the Holy Grail is fiction, they would be on rather firmer grounds as far as the case is concerned - but would shred their general credibility, which is not what they want.
If they win, it will be a major change to UK copyright law, and a disaster for all writers of historical fiction, who potentially could get sued by any non-fiction writer who had published on the same subject. But I don't think they will. This piece from theBookseller.com, better informed on the niceties of copyright law than me, thinks that they're on dubious ground. (And note the explanation of the successful case brought involving James Herbert's The Spear and Trevor Ravenscroft's The Spear of Destiny, which shows that it's not as relevant to the present case as some reports have implied.) Moreover, Baigent is not performing well on the stand under cross-examination from Random House's lawyer John Baldwin. And Mr Justice Peter Smith, who has read both books (which, as a friend with some legal training said "by all accounts will spoil his weekend nicely"), seems pretty clued up.