Gently heat the caster sugar with the water in a pan. When the sugar has dissolved, increase the heat and boil the mixture for 2 minutes.
Place the blackcurrants in a blender and whizz to a purée. Add the prepared syrup, lemon juice and crème de cassis to the blackcurrant purée and whizz again.
If you have an ice cream-making machine, transfer the mixture into it and follow the