Add the frozen berries, water and sugar in a medium saucepan and bring to boil without stirring. Mix together the potato flour and water to form a smooth runny paste and add slowly and stirring into the berry mixture. Let the soup come “to bubble”, and then take off the heat. Spoon into serving bowls and serve on its own or with porridge, you could even try it with a bit of natural yoghurt. If you’re not serving the soup immediately, sprinkle the top with a bit of caster sugar to avoid a “film” forming on the top.