Using sharp knife, pierce 12 cuts into lamb; press garlic and oregano into cuts.
Place lamb in large flameproof baking dish; pour wine, stock and juice over lamb.
Cover tightly; roast lamb in hot oven 30 minutes then reduce temperature to moderate; roast lamb, covered tightly, brushing occasionally with pan juices, for a further 3 hours or until lamb is extremely tender.
When lamb has about 1 hour left to roast, place potato, lemon and olives in separate baking dish; drizzle with oil, and place in oven; roast, uncovered, until browned and tender.
Transfer lamb to serving dish; cover to keep warm.
Heat 1 tablespoon of the reserved lamb juices in same flameproof baking dish; stir in flour.
Cook, stirring, until mixture is well browned.
Gradually stir in remaining lamb juices and the water; stir until gravy boils and thickens.