Pick over the oysters for shells. Simmer the oysters and oyster liqueur in a covered pot over low heat with a little salt and black pepper. Stir occasionally until the edges of the oysters curl, about 6 minutes.
Lift the oysters out of the stock with a slotted spoon and put them in a colander with a bowl underneath to catch the pot liquor. Pour the stock from the pot into the bowl, being careful to discard any oyster dregs. Set the stock aside.
Cook the celery and onion in a little water until tender; drain, reserving the broth. Add the broth to the oyster stock (should measure about 2 cups), and then add the milk and cream (reserve a little cream to whip for garnish). Heat the butter, then whisk in the flour to make a roux. Add the stock mixture to the roux, and stir briskly until thickened and smooth. Add a little nutmeg, a pinch of curry powder, the lemon juice, Worcestershire, and cayenne. Correct the seasonings.
Mash the oysters with the celery and onions and mix well; add to oyster stock. Keep warm over hot water until time to serve. Whip the reserved cream. Pour bisque into individual bowls. Swirl an even portion of whipped cream into each bowl. Add a little paprika and chopped parsley to garnish. Offer sherry, if desired.