Preheat the oven to 200°C (180°C fan oven) mark 6. Put the guinea fowl in a bowl, add the lemon zest and juice, bay leaves, thyme and peppercorns. Cover and leave to marinate for 1hr. Put the bird in a roasting tin, breast side down, stuff with the lemon quarters and the butter, pour the stock over and roast for 50min. Turn the guinea fowl on to the backbone, and continue to roast for 20min or until the juices run clear when the thigh is pierced with a skewer. Put the guinea fowl on a board and cover with foil. Leave to rest.