Place the berries, sugar and 200ml water in a small saucepan and heat gently until the sugar has dissolved and the blackberries are soft. Mash the blackberries with the back of a wooden spoon then push the mixture through a fine sieve into a measuring jug. Add more hot water to the jug so that the total liquid quantity is 500ml (2 cups).
Return the liquid to a clean saucepan and sprinkle over the gelatine. Place over low heat and stir until the gelatine has dissolved. Remove from the heat and allow to cool for a few minutes, then pour into four.
125ml (½ cup) glasses or bowls and chill for 4 hours, or until set. Serve with vanilla ice-cream.
In a large bowl whisk together the sugar, cream, vanilla extract and lemon juice with electric beaters until soft peaks form and the mixture is still pourable (do not reach stiff peaks as the resulting ice-cream will have ice crystals).
Gently fold through the sour cream until well combined. Transfer to a shallow tub and freeze for a minimum 4 hours, or until the ice-cream is solid. Remove from the freezer about 15 minutes before serving to allow to soften.