Puree the berries with the icing sugar and the vanilla seeds. Dissolve the gelatine in the water, mix together with 2 tbsp of the mixture and immediately stir into the remaining mixture. Stir in the yoghurt.
Fill into glasses
Fill the glasses with the mousse and biscuits in alternating layers, cover and leave to set in the fridge for approx. 2 hrs.
Decorate the trifle with berries.