Soak the gelatine in cold water until softened. Put 100ml water and ¼ 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 it into the liquid. Strain into a bowl, leave until cooled then 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.