Puree the raspberries and strawberries. Press through a sieve into a bowl. Add the sugar and lemon juice. Chill. To serve, stir the mixed fruit into the puree. Spoon into soup plates. Scatter the mint over the top. Set the creme fraiche on the table so everyone can add a dollop. Pass the bottle of Champagne and let everyone pour a glug into his bowl. Pour the rest of the bubbly into glasses.