You will need a heavy pan with a well-fitting lid (not aluminium, because of the vinegar). Place the shanks and marinade in it, adding a healthy pinch of salt. Cover and place in a medium to gentle oven, and cook for approximately 3 hours, turning the shanks every 30 minutes. If they are cooking too fast, turn the oven down: the secret is slow and low with this dish. The shanks want to be thoroughly giving, but still just holding on to the bone. When this is achieved, remove the shanks and keep warm. Add the glass of port to the juice, place it on the heat, and reduce until your sauce is to your satisfaction. Pour it over the shanks through a sieve (to remove the spices) and serve.