Peel and roughly chop the garlic cloves and melt the butter in a small pan. Mix the butter, garlic, salt and rosemary along with a good twist of pepper in a bowl and then rub this mixture all over the lamb. Place the lamb in a roasting dish and leave to stand for one hour before cooking. Spread any remaining marinade mixture over the lamb, then cook in a preheated moderate oven (Mk 5 - 375ºF - 190ºC) for 45-50 minutes, basting frequently in the juices.