Anna's recipe calls for cooking the lamb for at least five hours, regardless of size, but when we tested this recipe in the U.S., we discovered that the cuts available in most markets here aren't quite as rich as the cuts she cooks with in London. For smaller cuts in the U.S., make sure to baste frequently, and start checking the texture of the meat starting around hour three. Pull the roast from the oven when it's cooked through but still nice and juicy.