1) In a large bowl, sprinkle the gelatine over 250ml of the juice and allow to stand for a few minutes. Meanwhile, heat the remaining 750ml of juice until just boiling and pour it over the juice and gelatine mixture. Stir until the gelatine is dissolved.
2) Pour into a 20x20cm pan and chill until firm, about 3 hours or overnight. Dip the pan briefly into hot water to loosen,
Cut into squares or shapes, and serve.