Separate the eggs, putting the yolks into one basin and the whites into a second. Add the sugar to the yolks along with the finely grated lemon rind and the lemon juice. To get the right lemon flavour, you need about
Measure the water into a saucepan and sprinkle in the gelatine. Allow to stand for a few minutes, then place over low heat and stir until the gelatine has dissolved. Draw off the heat. Holding the pan well above the mixing basin, add the gelatine to the lemon mixture, pouring in a thin, steady stream. Whisk all the time and when the gelatine is blended in set the mixture aside to cool until beginning to thicken and set.
Stiffly beat the egg whites and fold into the lemon mixture. Pour into a serving dish and chill until set firm. Spoon the lightly whipped cream over the mousse about 1 hour before serving. Chill until ready to serve.