Tip the sugar into a measuring jug and pour over boiling water to the 400ml mark. Add the cinnamon stick, broken in two, and stir to dissolve the sugar.
Put the fruits and cognac into a pan, pour over the syrup and bring to the boil. Simmer, partly covered, for 15 mins, then remove from the heat and leave to cool for a few minutes if serving warm, or chill if serving cold. Serve with some crème fraîche or ice cream.