Couple of glugs of oil in a large saucepan. Add the onion and leek until soft, and then add the finely chopped celery, carrots and apple. Cook on a low heat stirring occassionally for about 10 minutes. Add the ginger, paprika, cayenne and basil leaves, and then pour over the stock. Bring to the boil then simmer for around 40 minutes or until the vegetables are soft. Process the soup in a blender until smooth. Season with salt and pepper if needed. Serve in warmed bowls, garnished with the celery leaves and a pinch of paprika.