The Elixir of Death by Bernard Knight
Prince John still plots to seize the throne from his brother, Richard the Lionheart - and in his wicked schemes he is actively supported by King Philip of France who offers to help John financially by sending him a mysterious alchemist, a Mohammedan named Nizam, who claims to be able to turn base metals into solid gold. But the ship which was transporting Nizam and his retainers to the Devon tin mines is found wrecked off the south Devon coast, its crew savagely slaughtered. Shortly afterwards, Sir Peter le Calve, a Norman knight living near Exeter, is foully murdered, being part-crucified and beheaded, his head stuck on the roodscreen of the cathedral. It's up to Sir John de Wolfe, the county coroner, to find a motive and connection between the killlings. And just what is his unscrupulous brother-in-law, the disgraced ex-sheriff and known Prince John-sympathiser, Richard de Revelle, trying to hide?