Put the clams and 4 tablespoons of the fish stock in a large saucepan. Cover, bring to the boil and steam for about 5 minutes or until the shells open. Remove the clams with a slotted spoon; strain and reserve the stock. When cool enough to handle, remove the clams from their shells and chop them coarsely.
Cook bacon in the saucepan over moderate heat for 3 minutes or until crisp. With a slotted spoon, remove and reserve to garnish.
Add the onions to the pan together with 4 tablespoons of the stock. Sauté for about 5 minutes or until soft but not browned.
Stir in the potatoes, remaining fish stock and reserved clam stock. Bring to the boil. Simmer, partly covered, for about 10 minutes or until the potatoes are very tender.
Remove from the heat. Purée 2 ladlefuls of the vegetables in a blender or food processor until smooth. Add the purée to the soup and return to the heat.
Stir in the milk and the chopped clams. Simmer for about 5 minutes. Add the pepper and parsley and serve, garnished with the bacon, if used.