Finely grate the rind of 1 grapefruit, making sure you don't get any of the bitter white pith. Cut the skin and the white membrane off both grapefruit and cut between the membranes to remove the segments. Put them in a large dish, sprinkle with 2 tablespoons of the sugar and set aside.
Whisk together the cream and yogurt until thick but not stiff.
Fold in the elderflower cordial, spices, grapefruit rind and remaining sugar until smooth. Pour the mixture into attractive glasses, arranging the grapefruit segments between layers of parfait.
Sprinkle the top with sugar and serve immediately with brandy snaps, if liked.