Soak the gelatine in a bowl of cold water. Put on the kettle and when it boils pour ½ litre of the boiling water over the mint leaves and sugar.
Leave to infuse for 5 mins then strain into a large jug. Squeeze the excess moisture from the soaked gelatine then stir it into the hot mint mixture until dissolved. Then add the grapefruit juice. Set aside to cool.
Cut the peel and pith from the grapefruit with a sharp knife then cut between the segments to release them, reserving any juice.
Cut the segments into about 3 pieces each then distribute the grapefruit and raspberries between 6 glasses. Stir any grapefruit juice into the mint jelly then pour it into the glasses and chill until set.