Pre-heat the oven to 375°F (190°C).
Crush the garlic and sea salt to a paste in a pestle and mortar. Add the oil, the chopped rosemary leaves and a good seasoning of pepper. Mix well.
Spread a large sheet of foil over a roasting tin. Place the lamb on it and, with a skewer, stab the fleshy parts of the joint several times. Spread the rosemary mixture all over the upper surface of the lamb and tuck in the sprig of rosemary.
Bring the edges of the foil up over the lamb, make a pleat in the top and scrunch in the ends. Ensure the parcel is loose to enable the air to circulate. Place the lamb in the oven and bake for 2 hours.
At the end of 2 hours, open out the foil, baste the joint well with the juices and return it to the oven for a further 30 minutes to brown. This should result in meat that is very slightly pink. If you prefer, cook it for more or less time.
Meanwhile, make the sauce by combining the redcurrant jelly and vinegar in a small saucepan. Whisk over gentle heat until the redcurrent jelly melts into the vinegar. Add the chopped mint and season with salt and pepper to taste. Pour into a jug and set aside until the lamb is ready to be served.
Once the lamb is cooked, remove it from the oven, spoon off the fat and reserve the juices left in the tin to make gravy. Meanwhile, cover the lamb loosely with foil and allow it to rest for 20 minutes before carving.
For the gravy, add the white wine to the reserved juices, stir and let it bubble until it has become syrupy. Season with salt and pepper to taste and pour into a warmed gravy boat.
Serve lamb with gravy and offer Redcurrant and Mint sauce separately.