Shanks are from the bottom section of the leg just below the knee. They can be 'french trimmed' which is where a small bit of meat is removed from the bone to make the shank look more pleasing. You will need one shank per person.
Shanks are best cooked long and slow until the meat literally falls off the bone. The gelatinous sinews of this small joint create a rich thick sauce when you pot roast them in wine or stock at Gas mark 3, 150ºC, 325ºF.
If you're looking for an inexpensive cut to cook long and slow, the scrag can also be cooked well in liquid.