Put the gelatine sheets in a cup, filled with ice cold water. Leave for about 10 minutes.
make the yogurt mixture
Put one cup of water into a saucepan and place it over medium-high heat. Add the yogurt, caster sugar and limoncello into the bowl. Place the bowl over the top of the saucepan, make sure that the bowl doesn't touch the water any time during the process. When the sugar dissolves and the mixture is warm, add the dry gelatine sheets. Combine everything using a spatula and set aside.TipWring gelatine sheets gently to remove any water.
In another bowl mix together the whipping cream and icing sugar, to get a soft peak.
Add 1/3 of the whipping cream to the yogurt mixture and combine well, using a spatula. Fold gently the rest of the whipping cream, so that you get a light and airy mousse. Pour the mousse into 4 glass cups and refrigerator for about an hour. Serve with rhubarb granita or fresh seasonal fruit.