Gently heat the sugar and 500ml/18fl oz water until the sugar has dissolved. Bring to the boil, stirring occasionally. Remove from the heat and allow to cool.
Stir the coffee and sugar syrup together, put in a freezer container and freeze until solid.
Just before serving, turn the granita out of the freezer container and chop into chunks. Put in a blender or food processor, and briefly whiz until it forms small crystals.
Spoon the granita into tall glasses, and serve immediately