Crush the garlic and the sea salt together in a pestle and mortar until pureed. Add the oil, chopped rosemary and a good seasoning of pepper. Mix.
Spread a large sheet of foil over a roasting tin. Place the lamb on it and stab the fleshy parts of the joint several times with a skewer. Spread the rosemary mixture all over the upper surface of the lamb and tuck in a sprig of rosemary.
Bring the edges of the foil up over the lamb and scrunch the ends together. Bake for 2 hours, then open out the foil, baste the joint well with the juices and return to the oven for 30 minutes to brown.
Meanwhile make the sauce. Combine the redcurrant jelly and vinegar in a small saucepan and whisk over a gentle heat until the jelly melts. Add the chopped mint and seasoning and pour into a jug.
When the lamb is cooked remove from the over and rest for 20 minutes before carving. Discard the foil, spoon off the fat and make some gravy with the remaining juices. Add the white wine and stir and bubble until syrupy then tip into a jug.
Serve the lamb with the gravy and sauce, some new potatoes and vegetables of your choice- i like runner beans when is season.