Trim any excess fat from the lamb. Combine the sauce ingredients in a saucepan. Bring to the boil, cover and simmer for 20 minutes. Place the lamb in a heavy flameproof casserole or pan and pour over the sauce. Bring to the boil, cover and simmer gently for 1 to 1 ¼ hours, turning the meat halfway through cooking. Remove from the heat and allow the meat to cool in the sauce. When it is cold, set aside for a further 3 hours, or leave in the refrigerator overnight.
Lift the meat out of the sauce and place on a rack in a roasting pan. Roast in a preheated moderate oven (180°C/350°F, Gas Mark 4) for about 1 hour until the lamb is tender.
To prepare the dips, place the plum sauce and hoisin sauce in separate dishes. Mix the soy sauce with the wine vinegar in another dish. Slice the lamb and arrange on a serving dish. Serve, hot or cold, garnished with shredded scallion and ginger, with dips.