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). When the meat is done to your liking, remove to a serving dish, cover with foil or the dish's own cover, and leave to rest for about fifteen minutes. It is important the meat be allowed to rest; you can also make gravy from the pan juices during this time.