Prepare the syrup by placing the raspberries, sugar, water and lemon juice in a large saucepan. Bring to the boil over a moderate heat and cook for 2-3 minutes. Remove and allow to cool a little before pressing through a sieve, collecting the syrup underneath in a bowl.
Allow to cool to room temperature, then cover and chill until ready to serve. When ready to serve, pour 35ml of the syrup into the base of each glass and top with soda water and ice. Stir well and garnish with a slice of lemon on the rim of the glass.