Not so long ago, casually throwing the Knights Templar into polite conversation was a litmus test of mental health. One of Umberto Eco’s characters in Foucault’s Pendulum summed it up perfectly. He declared that you could recognise a lunatic "by the liberties he takes with common sense, by his flashes of inspiration, and by the fact that sooner or later he brings up the Templars". But all good things come to an end. The enigmatic medieval monk-knights are no longer a fringe interest for obsessives. They are now squarely mainstream. And as 18 March 2014 draws closer, Templarmania is going...