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