Put the wine to chill well in advance. Measure the water into a saucepan, add the sugar and thinly pared rinds from the oranges and lemons. Stir over low heat to dissolve the sugar, then bring to the boil. Simmer for a few moments, then strain and leave until cold.
Squeeze the juice from the oranges and lemons and add to the cold syrup. When ready to serve, put a few ice cubes in the serving jug. Pour in the chilled wine and stir well to blend. Top up with the soda water, and float slices of orange and a sprig of mint.