Dilute the elderflower cordial with 570ml/1 pint cold water. Pour 4 tbsp into a small pan. Sprinkle over the gelatine and set aside for 10 minutes.
Arrange the raspberries, blueberries and grapes into four whisky tumblers or wine glasses.
Place the pan over a very gentle heat and swirl the pan until the gelatine has completely dissolved. Do not allow the gelatine to boil, as this will prevent the jelly from setting. Pour in the remaining diluted cordial and stir well over a low heat. Pour into the glasses.
Chill for 3 hours or until softly set. Just before serving, spoon over a little extra cordial.