Put the sugar and lemon zest in a small pan with 575 ml water. Heat and stir until the sugar dissolves. Increase the heat and simmer, uncovered, for 15 minutes. Cool, then remove the lemon zest.

Peel, core and slice the apples. Add to the pan and cook for another

minutes, until tender. Leave to cool in a bowl.

Put the cooled apples and syrup in a blender or processor, add the lemon juice and whizz until smooth. Freeze for 1 hour.

Remove from freezer and stir with a fork to break up the ice crystals.

Return to the freezer for another hour then stir again. Repeat 3 or 4 times until the right texture is reached, then freeze until needed.