Line a 23cm x13cm| 9 x-5" loaf tin with a double layer of cling film.
Blend the blackberries and raspberries in a food processor until just broken up. Press through a sieve into a bowl, to extract as much liquid as possible.
Heat the sugar, water and salt to a boil in a pan, stirring until the sugar is dissolved. Boil for 2 minutes. Remove from the heat.
Whisk the egg whites with an electric whisk in a large bowl until foamy. Slowly pour the hot sugar syrup into the egg whites, whisking for 8-10 minutes, until the egg whites are cooled to room temperature.
Whisk the cream until thick.
Fold the berry puree into the egg white mixture until almost blended, then fold in the whipped cream.
Spoon evenly into the tin and smooth the top. Cover with foil and freeze for at least 6 hours, or overnight.
About 15 minutes before serving, remove the semifreddo from the freezer. Place on a serving plate and stand for 10 minutes to soften slightly. Decorate with raspberries and blackberries.