1. Melt the butter in a large saucepan, add the onion and cook for 3 minutes, stirring. Add half of the stock to the saucepan with the chopped tomatoes and bicarbonate of soda. Bring the soup to the boil, then reduce the heat and simmer for 20 minutes. Remove from the heat and set aside to cool.
2. Transfer the cooled soup to a food processor or blender and blend into a puree. Pour through a fine strainer back into the saucepan.
3. Add the remaining vegetable stock and the fusilli to the saucepan, and season to taste. Add the sugar to the saucepan, bring to the boil, then reduce the heat and simmer for 15 minutes.
4. Ladle the soup into a warmed tureen, swirl the double cream over the the surface and garnish with fresh basil leaves. Serve immediately.