In a large pot over medium heat saute leeks in butter. Remove from heat, let cool a bit. Stir in flour. Mix well. Add water and mix well again. Return to heat and add potatoes, salt, and pepper. Bring to a boil. Then lower heat, cover and simmer for 30 minutes. When potatoes are tender break up potatoes with a masher, add milk and heat just under boiling. Garnish individual serving dishes with butter and chives and serve with a large bowl of crusty croutons.