Put everything, other than jelly, together into a freezer bag, tie securely, and leave in the refrigerator to marinate overnight.
Preheat the oven to 400 degrees F.
Take the lamb in its package out of the fridge to come to room temperature.
Put the lamb shank along with its marinade into a roasting tin and cook for 1 to 1 1/2 hours, depending on the size of the shank. Turn the shank over halfway through cooking. By the time it is done it should look bursting with bronze color; remove from the pan and let stand 5 to 10 minutes before serving.
While it is resting, put the roasting tin on the stove and stir in a little water and 1/2 teaspoon red currant jelly to make a light gravy. Slice lamb and drizzle with gravy.