1) Make the raspberry syrup: in a small saucepan, whisk together the orange juice and cornflour until smooth. Add the raspberries, raspberry jam and granulated sugar. Bring the mixture to a boil over medium-low heat, stirring frequently. Reduce the heat to medium-low and simmer for 5 minutes. Pour the mixture through a fine-mesh sieve into a small bowl, pressing on the solids with a wooded spoon to essence as much liquid as possible. Cool the syrup for 30 minutes.
2) Using an ice cream scoop, put 120ml of sherbet into each glass. Scoop 120ml of vanilla ice cream on top of the sherbet. Pour 80ml club soda on top of the ice cream. Drizzle each glass with 2 tbsp of raspberry syrup and garnish with fresh raspberries. Serve immediately with straws and sundae spoons.