Put the biscuits in a plastic food bag and crush using a rolling pin. Melt the butter in a medium pan, then stir in the biscuits. Tip this mixture into a loose-bottomed 24cm round cake tin, about 5cm deep and press down evenly. Chill.
Make the mousse: submerge the gelatine leaves in a large bowl of cold water and leave to soak for 5 minutes. Heat the single cream in a small pan until it just comes to the boil. Take off the heat. Lift the gelatine leaves out of the water and give them a good squeeze to get rid of the water. Stir into the hot cream one by one – they will dissolve immediately. Leave to cool slightly.
Beat together the fromage frais, sugar, orange zest and juice with a whisk or wooden spoon. Stir in the cooled cream and gelatine mixture and the raspberries. Crush just a few of the raspberries against the sides of the bowl and stir again to ripple through, leaving most whole.
Whip the double cream lightly to soft peaks, then fold it into the raspberry mixture. Pour into the cake tin and leave to chill for at least 3 hours, or overnight, until set.
To make the sauce, blitz all the ingredients in a food processor or blender. Chill until ready to serve. (The sauce and the mousse can be made up to a day ahead).
To serve, run a knife between the mousse and the tin, lift it out of the tin and take it off its base. Put on a serving plate and pile a mixture of fruits on top to decorate. Dust with icing sugar and serve with the sauce.