Break the celery stems apart and remove the tougher strings from the outer parts of the celery. Chop the celery and leeks into slices.
Put the chopped leeks and celery into a saucepan with the butter. Gently fry (sweat) the leeks and celery.
Add the flour to the leeks and celery, stirring so that the flour coats the vegetables evenly. Gently fry the mixture for a minute. Keep to a low temperature to avoid browning the flour.
Add the water to the celery and leeks, stirring into the flour mixture and heat until the water is gently simmering. Continue to heat the soup so that it simmers continuously for about 45 minutes.
Using a blender (or hand mouli) blend the leeks and celery together to create a creamy mixture. Season with salt and pepper.
Ladle the creamy Celery and Leek Soup into bowls and serve with croutons (sippets). Add a dash of cream for extra richness.