Soften the onion, celery and leek in the butter without colouring and add the curry powder. Add the wine and reduce by two thirds. Turn up the heat to medium, add the mussels, cover and cook for five minutes until the mussels have opened. Strain, reserving the mussel juices, and leave to cool. Return the liquid to the pan and add the stock, thyme, cayenne pepper and saffron. Simmer for 5 minutes, add the cream and potato, and simmer for another five minutes. Blend until smooth, and season. Remove the mussels from their shells and gently warm in the soup. Divide the mussels between the bowls, pour over the soup and sprinkle with chopped chives.