Preheat the oven to 180°C.
Drain and finely chop the stem gingers (keep the syrup).
Mix together the ginger, rosemary, garlic and oil in a bowl.
Make several slits over the surface of the lamb, then rub it with the rosemary and ginger mixture, pushing this into the slits. Season with salt and pepper, then put into a roasting dish.
Roast for about 90-120 minutes, depending on how well-done you like lamb (and baste occasionally with the juices.)
30 minutes before the end of cooking add the plums and the port. 15 minutes before the end of cooking brush the lamb with the stem ginger syrup.
Remove the lamb from the and rest for 15 minutes. Keep warm he plum and the jus in the oven.
Carve the meat and serve with the plums and pour over the jus.