Melt the buttere in a saucepan, simmer the vegetables, except the potatoes, until soft but not browned. Add the potatoes and cook over a slow heat, uncovered, for a few minutes. Stir in the stock aand water and milk mixture. Season with salt and pepper, then cover the pan and simmer for 15 minutes. When cool, process the soup in a blender until smooth. If the soup is too thick add a little iced water. Chill the soup well before serving with chopped chives on top.