Wash the clams under cold running water.
Put the aromatics - chopped onion, leek, carrot, parsley and thyme into a large pot, add 400ml/14fl oz of water and simmer for 10 minutes to infuse the flavour. Add the wine, and bring to a boil.
Add the clams, cover and cook, boiling quickly until the clams open, this just takes a minute or two. Drain them into a colander, being sure to catch the cooking liquid in a bowl underneath. This will be used in the chowder.
Set aside while you pull the clams from their shells. Discard the shells, and the sprigs of parsley and thyme. Keep the clams and the chopped vegetables, the aromatics, and place them in to the reserved broth.
To make the chowder, sauté the onion in two tablespoons of unsalted butter, over medium heat in a large pot until it is soft and transparent. Add the water or stock and bring to a simmer.
Add the potatoes to the pot, and simmer gently until the potatoes are cooked.
Take half of this potato mixture and blend in a food blender, or food processor. This will give you a thick, creamy soup which will thicken the whole chowder.
Combine the two potato mixtures, the clams in their juices and cooking broth, and the cream, and bring to a simmer.
Taste for seasoning, and serve at once, garnishing with the chopped parsley, and if desired, a dollop of whipped cream.