Bring the sugar and two cups of water to the boil, stirring until the sugar has dissolved. Simmer until the liquid has become slightly syrupy and reduced to one cup. Remove from heat.

Combine the coffee and extra water and add the sugar syrup. Mix well. Pour into a shallow metal tray and place in the freezer. After 1 1/2 hours, break up the coffee ice crystals with a fork, stir and return to the freezer. An hour later, fork it through again, and again an hour later, until it's frozen but with a slushy granita consistency. Serve in glasses.