Pour 150ml of cranberry and raspberry juice into a small heatproof bowl, sprinkle over 3 teaspoons or 1 packet of powdered gelatine and leave to stand for 5 minutes. Stand the bowl in a saucepan of water and heat for 5 minutes until the gelatine has melted to make a clear liquid.
Stir in the remaining cranberry and raspberry juice then the sliced strawberries. Spoon into the bases of 6 250ml individual jelly moulds, set on a tray. Allow to cool then freeze for 15 minutes or until the jelly has just set.
Meanwhile, add 4 tablespoons of water to the rinsed heatproof bowl, sprinkle over the remaining gelatine and leave to soak for 5 minutes. Stand the bowl in a saucepan of gently simmering water and heat until it makes a clear liquid.
Pour the custard into a separate bowl, lightly whisk in the cream then the warm gelatine in a thin steady stream. Pour this over the set strawberry jelly and chill in the fridge for 6 hours or overnight until set firm.
To serve, dip the moulds into a large dish of hot water, one at a time, count to 10 then lift out of the water. Dip a finger in a little water and loosen the top edges of the jelly. Invert the mould on to a plate or small cake stand, jerk it to release jelly and finally remove the mould. Repeat with other jellies and serve.