1. Peel and chop potato, onion and leeks.
2. Put the potato, onion, leeks, drained butter beans and water in a large saucepan.
3. Boil until potato and vegetables are soft.
4. In a food processor or sieve, blend half the potatoes and vegetables with the cooking water and all the milk until smooth.
5. Add the remaining potato and vegetables to the blended soup mixture.
6. Season to taste.
7. Put the soup back in the large saucepan and bring to the boil before serving.