Place the blackberries, ginger, sugar and 300ml (½ pt) water into a saucepan and simmer over a gentle heat for 15 minutes until the blackberries have broken down slightly. Push the mixture through a sieve into a jug, getting as much of the pulp through as possible.
Pour a little boiling water into a cup and add the gelatine. Stir well until the gelatine has dissolved.
Top up the liquid in the jug to 500ml (18fl oz) with boiling water and then stir in the gelatine mixture.
Pour into 4 serving glasses. Top with crème fraîche (optional) and reserved berries. Leave to cool and then refrigerate for 1-2 hours until set.