To a large pot add potatoes, 4 of the leeks, celery, onion, bay leaf and chicken stock. Let boil until potatoes are soft. Chop the remaining 4 leeks. Mix flour and butter in small bowl to make a roux for thickening the soup. Add the remaining leeks, roux, cream, fresh thyme, and salt and pepper, to taste. *Blend soup with an immersion blender (or in batches in a blender or food processor*). Simmer for 20 more minutes. Remove bay leaf and serve.
*When blending hot liquids: Remove liquid from the heat and allow to cool for at least 5 minutes. Transfer liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.