Heretic by Bernard Cornwell

Heretic (The Grail Quest, Book 3) - Thomas of Hookton, seasoned archer for England is sent to capture Astarac, ancient castle of his ancestors.  The Castle is rumored to conceal the Grail.  Thomas learns of Genevieve who is to be burned as a heretic, and because he stops the proceedings, Thomas is now branded as a heretic.  Meanwhile, Guy de Vexille is manufacturing a false Grail.