1.Preheat oven to very slow 120°C (100°C fan-forced).
2.Bring a large saucepan of water to a boil, add lamb, simmer 15 minutes. Drain, pat lamb dry. Pierce lamb all over with sharp knife press salt into cuts.
3.Heat butter and oil in large flameproof dish, cook lamb, turning, until browned all over. Add wine, vinegar, port, stock and garlic to dish. Roast lamb, in oven, covered, 4½ hours.