Put tomatoes into a bowl of boiling water for 30 seconds, then refresh in cold water. Drain.
Peel and discard the skins and quarters the tomatoes. Put them in a large saucepan and pour in the chicken or vegetable stock. Bring to a boil, reduce the heat, and cover a simmer gently for 10 minutes, until the tomatoes are soft.
Stir the tomato paste, vinegar, sugar and basil. Season with salt and pepper, then cook gently, stirring for 2 minutes. Puree the soup in a blender of food processor then return to the saucepan and reheat until piping hot.