Sieve the juice into a jug and pour it back into the pan, heat gently and add the sugar in 50g amounts until the juice is sweet enough. Stir until the sugar dissolves. Soak the gelatine leaves in cold water for five minutes then lift them out and stir them into the warm juice. Divide the juice between 4 small tumblers (don’t worry if you don’t fill them up) and chill until set. to serve, pour a thick layer of custard onto each jelly.