In a large heavy frying pan, combine 800g of the raspberries and 2/3 cup of the sugar. Stir over medium heat for about 2 minutes, or until the sugar melts and the raspberries begin to exude their liquid. Simmer for about 5 minutes, or until the raspberries have broken down. Stir in the Chambord. Strain the mixture through a fine sieve, pressing on the solids with a ladle to exude as much liquid and puree as possible. Discard the solids. Set the raspberry sauce aside to cool completely.
In a large bowl, using an electric mixer, beat the cream and remaining 1/2 cup sugar until soft peaks form. In another large bowl, beat the creme fraiche until it thickens. Fold the whipped cream into the creme fraiche.
Drizzle 1/3 cup of the raspberry sauce over the bottom of a 3 1/2-litre trifle bowl or another flat-bottomed glass bowl. Arrange a single layer of the sponge finger biscuits (about 6) over the bottom of the trifle bowl. Drizzle 1/3 cup of the raspberry sauce over. Spread about a quarter of the cream mixture over the biscuits. Arrange 12 raspberries over. Drizzle 1/3 cup of the raspberry sauce over. Repeat layering more biscuits, sauce, whipped cream mixture, and fresh raspberries 3 more times, finishing with the remaining cream mixture. Cover and refrigerate for at least 4 hours to allow the sponge finger biscuits to soften and the flavors to blend.
Scatter the remaining berries over the trifle. Dust with icing sugar and serve.
