Shell prawns, remove back vein. In a saucepan melt butter, add peeled and diced onion, chopped carrot and celery; cook, covered, 10 minutes. Add wine, water, rice and undrained, mashed tomatoes. Bring to boil, reduce heat and simmer covered 45 minutes. Remove from heat. Blend soup, in several lots, in blender. Return to saucepan, season with salt and pepper; add finely chopped prawns, drained and flaked crab, and cream. Heat through gently for about 5 minutes.
To do ahead
Soup can be prepared the day beforehand (but do not add prawns, crab or cream until reheating). Reheat gently, stirring; if soup thickens on standing, add a little more cream.