Strain the syrup from the strawberries into a large heatproof bowl. Sprinkle the gelatin over it and stir well. Stand the bowl in a pan of hot water and stir until the gelatin has dissolved.
Let cool, then chill for just under 1 hour, until beginning to set. Meanwhile, cut the sponge cakes in half lengthwise and spread the cut surfaces with the jam.
Carefully whisk the creme fraiche into the strawberry jelly, then whisk in the canned strawberries. Line a deep, 8-inch loose-bottomed cake pan with waxed paper.
Pour half the strawberry mousse mixture into the pan, arrange the cakes over the surface, and then spoon over the remaining mousse mixture, pushing down any sponge cake that rises up.
Freeze for 1-2 hours until firm. Remove the cake from tha pan and carefully peel away the lining paper. Transfer to a serving plate. Decorate the mousse with whirls of whipped cream and strawberries.