Roughly chop three-quarters of the strawberries and put them in a food processor or blender with 300 ml (½ pint) boiling water and the sugar. Blend until smooth, then pour the mixture into a sieve set over a bowl and stir to allow the liquid to drip through.
Pour 200 ml (7 fl oz) of the grape juice into a heatproof bowl, sprinkle over the gelatine and allow to stand for 10 minutes. Place the bowl over a saucepan of simmering water and stir until the gelatine has dissolved. Leave to cool, then stir in the cassis (if used), strawberry liquid and the remaining grape juice.
Arrange the remaining strawberries in 6 large wine glasses, pour over the liquid and chill until the jelly has set.