Preheat the oven to gas 4, 180C, 160°C Pound together the ginger and leaves from half of the rosemary sprigs and mint. Add the oil and mix together. Make slits in the surface of the lamb and push in the ginger and herb mixture. Put onto a rack in a roasting tin and roast in the oven for 2 1/2 hours for medium meat (or around 3 hours for well done).
About 45 minutes before the end of the cooking time, add the potatoes to the roasting tin or a separate tray and drizzle with a little oil. Scatter with leaves from the remaining rosemary sprigs.
20 minutes before the end of the cooking time, glaze the lamb with 2 tsp syrup from a jar of stem ginger. Re-glaze with a further 2 tbsp syrup during the last 10 minutes of cooking time. Take out of the oven and leave to rest for about 15 minutes before carving. Serve with the roasted new potatoes.