Fry the potato and onion gently in the oil for a few minutes without browning. Add the leeks and about 140 mls (¼ pint) of the stock, together with the saffron and its infusing liquid, and cook until the vegetables are soft. Allow to cool slightly, make a puree in a blender or food processor, sieve, and stir in the rest of the stock. Chill until ready to serve. Just before serving, season to taste, and, if using, stir in the yoghurt or single cream.