Snap off the asparagus tips and set the stalks aside. Melt the butter in a deep pan with a cover. Add the asparagus tips, onion, leeks, celery and potato. Cover lightly and cook over the very lowest heat until vegetables are soft, 20 minutes or more. Meanwhile, place the chicken stock in a large saucepan with the reserved asparagus stalks, cut into large pieces. Bring to a boil and simmer, covered tightly, for about 30 minutes. Discard the stalks and set the resulting stock to one side. In a food processor, puree the softened vegetables and add to the asparagus stock. Season with the lemon juice, salt, white pepper and paprika, if desired. Allow to cool and refrigerate. Stir in the single cream and serve, garnished with a dollop of whipped cream, or Creme Fraiche and a sprinkling of paprika, if desired.