Rinse the clams under running water two or three times to remove as much grit as possible. Drain.
Melt the butter in a large saucepan over low heat. Add the flour and mix with a whisk or a wooden spoon and cook for 3 minutes. Set aside and allow to cool.
Add the wine, bay leaf and thyme to a large stockpot, bring to a boil and cook for 5 minutes over medium heat. Add the clams, cover and simmer for 5 - 7 minutes, or until the clams have opened. Strain, keeping the cooking liquid for later, and discard any clams that did not open. Set the remaining clams aside to cool. Once cooled, remove from their shells, chop up and set aside.
Strain the cooking liquid again through a fine sieve into the pan with the butter and flour mixture. Whisk together, place over medium heat and simmer for about 10 minutes, skimming the top twice, and then set aside.
In a large stockpot, heat the oil over medium heat and cook the bacon for 5 minutes, or until nicely colored. Reduce the heat, add the onion, cover and cook, without coloring, for 3 minutes. Add the celery and cook, covered, for 6 minutes, then add the potato and cook, covered, for 3 minutes. Pour in the thickened clam liquid, cover and simmer for 15 - 20 minutes, or until the potatoes are just done. Add the chopped clams and the cream and simmer for 5 minutes. Serve the soup sprinkled with the parsley.
Chef's tip: Shellfish can contain lots of sand and grit. Always rinse several times, using lots of running water.