Using a vegetable peeler, carefully peel the skin from the oranges and lemons, avoiding the white pith. Put in a saucepan with the sugar and 1 litre of tap water and bring to the boil. Simmer for 10 mins, then remove from the heat and leave to cool completely. Leave to stand, covered, for a few hours or, if you have time, overnight.
Meanwhile, squeeze the juice from the oranges and lemons.
When ready to serve, remove the peel from the sugary liquid and add the fresh juices. Add ice and lemon slices, then top up with the sparkling water to serve.