In a large pot, place potatoes, 4 of the leeks (reserving the rest), celery, onion, bay leaf and chicken stock and bring to a boil, about 15 minutes. Continue to 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. Remove bay leaf. Using an immersion blender (or in batches in a blender or food processor), blend soup until smooth. Pour into a medium pot and simmer for 20 more minutes until soup has thickened and serve.