Place the demerara sugar in a saucepan with 300ml/10fl oz water. Stir to dissolve, then bring to the boil and cook until it has the consistency of honey. Be careful to not let the sugar syrup caramelise and try not to stir too often as this may cause the sugar to crystallise.
Allow the sugar syrup to cool and then pour in the rosewater, lemon juice and a few drops of pink food colouring, if using.
Place a scoop of lemon sorbet into each glass with 3 ice cubes. Pour a quarter of the syrup into each glass and top up with sparkling water. Garnish with rose petals and serve straight away.