Squeeze the juice from the oranges into a measuring jug and make up to 300 ml (½ pint) with cold water. Put in a saucepan with the sugar, vanilla pod and cinnamon stick. Heat over a low heat, stirring, until the sugar has dissolved, then simmer gently for 5 minutes until a light syrup is reached.
Remove from the heat and leave to cool completely. Remove the vanilla pod and cinnamon stick.
Hull and halve the strawberries, then put in a large bowl with the remaining berries and pour over the syrup. Stir well and leave to marinate at room temperature for 30 minutes.