Heat half the oil in a saucepan and sauté the onion, peppers and courgette for 5 minutes until softened.
Pour over the stock, add the sugar and bring to the boil. Reduce heat, then cover and cook gently for 15–20 minutes. Add the garlic, allow to cool slightly, then tip into a blender or food processor and blend until smooth.
Add the vinegar and paprika, season to taste, then allow to cool. Chill in the fridge for at least 2 hours.
Serve the soup in bowls, drizzled with a little of the remaining olive oil and garnished with a few basil leaves.