Transfer to a small bowl, add the remaining ½ cup yogurt and stir to mix. Refrigerate for a minimum of two hours, if attainable, to permit the flavour to develop. When able to serve, pull the lamb meat aside with two forks. Turn the oven right down to 150°C (300°F) and roast, lined, for a further 4¾ hours . Roast the lamb and garlic heads, coated, for quarter-hour.