Heat the oil in a large saucepan and cook leeks for 5 minutes, or until soft. Add stock and potatos, bring to the boil, and simmer for 20 minutes, or until potatos have softened. Allow to cool a little, then puree the mixture with a bamix, or in the food processor. Add salt and pepper to taste, and serve hot with a generous sprinking of dried chives and sour cream or natural yoghurt if you desire. Delicous, warming, fast and easy!