You do know Brown laughs all the way to the bank!
And I suspect antiRepublicrat is correct: ideas are not copyrightable.
From the US Copyright Office
"Several categories of material are generally not eligible for federal copyright protection. These include among others: ... Ideas, procedures, methods, systems, processes, concepts, principles, discoveries, or devices, as distinguished from a description, explanation, or illustration."However, certain things aside from straight text are subject to copyright, such as the characters in a book. You can eventually cross a line in character development where you are infringing on the copyright of the author who created the character (yes, all fan fiction you read is infringement, but it's usually not sold so authors don't make a big deal about it, plus suing your fans is not a good idea as the RIAA has learned). He could make a claim that Brown is copying his character development of Jesus, but that's on pretty thin ice IMHO.
That's just my non-lawyer, but well-studied, opinion. This book has already survived two other lawsuits over the basic idea.
OTOH, this is in a UK court, so I can't be that precise. Their copyright law is pretty close to ours in this respect, but I don't know their precedent at all.
Many years ago Ben Hecht wrote a book about the plotters responsible for the Haymarket riots. It read like non-fiction, but Hecht had invented some composite characters to smooth out the narrative. Later, a screenwriter used Hecht's book as research for a screenplay and included some of these characters, assuming they were real people. As I recall, Hecht won that lawsuit. It's possible that Brown may be vulnerable since the material he used isn't historical fact, but someone's original interpretation of events.