Start the lamb in an oven preheated to 200C (about 400F) and cook uncovered for 25 minutes for each 500g (1lb) plus an extra 25 minutes.Baste each 25 minutes. When the lamb is cooking well, turn the oven down to 175C (350F).he general rule for lamb is, for every kilogram of weight, cook for an hour at 190 - 200 degrees Celsius, depending on the efficiency of your oven, or 180 degrees for a fan-f … orced oven. Thus, a 3kg leg of lamb will take three hours.