Begin with making the granita. Place the mandarin segments, sugar and lemon juice in a blender. Blitz on high speed until smooth. Strain puree through a fine mesh sieve set over a bowl. Discard any solids. Pour into an airtight container. Cover and freeze for 2-3 hours or until frozen and set.
While the granita sets, make the mousse. Place the gelatine in a sma