Soak the gelatine in cold water until softened. Put 100ml water and 1/4 of the Champagne in a pan with the caster sugar. Heat gently until the sugar has dissolved then squeeze the excess water from the gelatine sheets and stir into the liquid. Strain into a bowl, leave until cooled.
Add the rest of the Champagne and the cordial. Divide the jelly between 6 Champagne glasses. Chill for about 4 hours (or overnight) to set.