Put blackberries, sugar & lemon juice into a saucepan and simmer gently for 10 minutes. Press through a nylon sieve into a large mixing bowl.
Put the gelatin and 4 T of water into a heatproof bowl and soak for a few minutes. Stand the bowl in a saucepan of hot water and stir the gelatine slowly to dissolve. Pour into the blackberry puree whisking continually. When the mixture is beginning to set, fold in the 1/2 cup of lightly beaten cream.
Whisk the egg whites until the hold their shape, then fold into the mousse mixture. Pour into 6 or 8 individual serving dishes and chill until set. Garnish with the 1/2 cup of whipped cream and reserved whole berries.
Cooks Notes: Gooseberries, rhubarb, blackcurrants, and raspberries can all be used as an alternative to the blackberries, sweetening to taste.
The fruit can be made into a puree and frozen until required.