For more than two decades, Terry Pratchett has been regaling readers with tales of Discworld flat world balanced on the backs of four elephants, which are standing on the back of a giant turtle, flying through space. It is a world populated by ineffectual wizards and sharp-as-tacks witches, by tired policemen and devious dictators, by reformed thieves and vampires who have sworn to drink no blood. It is a world that is vastly different from our own… except when it isn’t.