1. Place the cherries into a saucepan with the caster sugar and the still water.
2. Bring the ingredients to boil.
3. Leave the mixture to simmer for 5 minutes until the cherries have started to soften.
4. Remove from the heat and allow to cool.
5. Place the mixture into a blender and blend until the liquid is smooth.
6. Place the liquid into a jug to serve and top with the sparkling water and stir well.
Keep refrigerated for up to two weeks.