In a small bowl mix together the ginger, rosemary, garlic and oil.
Place the lamb on a chopping board, season and make several slits over the surface of the joint. Stuff with the rosemary and ginger rub.
Season and place on a metal rack in a large non-stick roasting tin and roast for about 1½ to 2 hours, depending on desired 'doneness', basting occasionally with any lamb juices.
40 minutes before the end of cooking add the plums and the port. 20 minutes before the end of cooking glaze the joint with the stem ginger syrup.
Remove the lamb from the oven with half the plums and keep warm. To make the gravy: Spoon off any excess fat from the roasting tin and sieve the remaining plums, port and lamb juices into a pan. Season, if required.
Spoon a little over the lamb and serve the remaining sauce in a sauceboat.
Serve the joint with the remaining plums, roasted parsnips and seasonal vegetables.