In a shallow bowl, soak the gelatine leaves in cold water for about 5 minutes, until softened. Meanwhile, heat the cordial with the sugar over a low heat for 5 minutes, until all the sugar has dissolved. Strain the gelatine and gently squeeze out any excess water with your hands, then add to the sugary mixture. Heat to a simmer and cook for 3 minutes, until the gelatine has melted. Remove from the heat and leave to cool a little, then pour in the sparkling wine and stir to combine.
Divide half the raspberries into 8 dariole moulds, small glasses, or any other container that holds at least 180ml (6½fl oz). Pour in the liquid to fill one third of the container and chill for 2 hours. Add the remaining raspberries and top up with the rest of the liquid. (Doing this in two parts gives an even distribution of berries in the finished jelly.) Chill for 4 hours or overnight, until set.
Turn the jellies out onto a plate (dip the moulds into a bowl of boiled water to loosen before turning out) and serve with freshly whipped cream or Greek yogurt.