Sprinkle the vegetarian gelatin powder over the grape juice in medium-sized pot. Stir thoroughly until the powder is completely dissolved and the liquid appears to thicken just a little.

Put the pot on the stove and, over a medium flame, heat till just boiling. (About 200°F or 93°C by my candy thermometer.) Tiny bubbles will start appearing.

Pour the hot liquid into a heatproof rectangular casserole dish about 9x13 in or 23x33cm.

Leave to cool then cover with cling film and chill for at least 2 hours or until firm.

Cut into squares to serve.