The case of Michael Baigent and Richard Leigh vs Random House always promised Dickensian larks. This week, the whiff of Jarndyce and Jarndyce grew ever stronger in Court 61. In his defence against the suit brought by the authors of The Holy Blood and the Holy Grail, Dan Brown told of his helpmeet Blythe. He made his wife-cum-researcher, "deeply passionate about the sacred feminine", sound like a New Hampshire Esther Summerson as she selflessly sought out the occult theories that feed The Da Vinci Code.
In a nutshell, Brown's case is that there was an awful lot of it about (crackpot speculation on the bloodline of Mary Magdalene, the downfall of the Templars, the heresies of Leonardo, etc). And now there's even more. Simon & Schuster has just published The Secret Supper by Javier Sierra (£12.99). This Spanish chart-topper merely appears - because, M'Lud, I accept that Señor Sierra did his own original research - to splice The Name of the Rose and The Da Vinci Code into what Hollywood might deem the perfect pitch.
We're in Milan in the frozen January of 1497, as the grumpy giant Leonardo finishes off his "Last Supper" for the Sforzas. But a concealed assassin is finishing off the monks in the convent where the great mural will rest. A fire-and-brimstone brother called the Soothsayer hurls charges of heresy at artists, rulers and tonsured friends, while a Dominican detective arrives from Rome to solve all the seething mysteries. Is that odd Leonardo up to his thick (but suspiciously celibate and vegetarian) neck in a "a sinister occult plan... to install a pagan republic in the very heart of Italy?" Do his painted disciples, with their twisted poses and familiar Milanese faces, carry a grail-full of secret messages for posterity? Is Pope Alexander VI a Catholic? Well, for many of the characters here, probably not...
It hardly needs the holy inquisition to determine that The Secret Supper is a dog's dinner of scrambled scholarship, wild guesswork and paranoiac prose. And I (mea maxima culpa) rather enjoyed it. In Alberto Manguel, Sierra has a mischievous translator who makes merry with his breathlessly portentous style. Sierra knows enough to be interesting (from time to time) about the passion for occult symbols, pagan allusions and the mystic "arts of memory" that really did flourish in elite circles of Renaissance Italy. As for the elusive masterwork in Santa Maria delle Grazie itself, Sierra tries to steal a march on Brown and his buddies by basing his esoteric interpretation on the painting revealed after the complete restoration that ended in 1997.
Soon enough, The Secret Supper returns to familiar pseudo-historical ground. Say hello again to the ubiquitous descendants of Mary Magdalene, the hidden gospels of John, and, above all, our oldest heretical pals - the Cathars.
There is a genuinely illuminating book waiting to be written on the Cathar cult in modern times. For Sierra, as for so many others, the hierarchy-hating rebels represent the Jesus creed pure and simple. Foes of priestcraft, power and violence, they proclaim in the teeth of persecution the one subversive truth that "God is within us all and not inside a temple". Woodstock comes to 1490s Lombardy as New Age sentiments go on another recruitment drive through distant history. All in all, the image of Catharism found in books like Sierra's holds a mirror to a troubled age that craves togetherness but bridles at authority.
In the High Court, we await the wisdom of Mr Justice Smith. In the meantime, Brownites and anti-Brownites alike can get stuck into The Secret Supper. Very unlike the New England-Puritan DVC, it features a frisky, naked princess from a clan of heretics. Here, at least, is an author who knows how to put the doxy into heterodoxy.