Wash and dry the oranges; grate the rind on the finest part of a stainless steel grater. Put into a bowl with 2 eggs, 2 egg yolks and the castor sugar. Whisk to a thick mousse, preferably with an electric mixer. Put 3 tablespoons of water in a little bowl, measure the gelatine carefully and sprinkle over the water. Leave to ‘sponge’ for a few minutes until the gelatine has soaked up the water and feels spongy to the touch. Put the bowl into a saucepan of simmering water and allow the gelatine to dissolve completely. All the granules should be dissolved and it should look perfectly clear.
Meanwhile, squeeze the juice from the 2 oranges and 1 lemon, measure and if necessary bring up to ½ pint (300 ml/1¼ cups) with water. Stir a little of the juice into the gelatine and then mix well with the remainder of the juice. Gently stir this into the mousse; cool in the fridge, stirring regularly. When the mousse is just beginning to set around the edges, fold in the softly whipped cream. Whisk the 2 egg whites stiffly and fold in gently. Pour into a glass bowl or into individual bowls. Allow to set for 3–4 hours in the fridge.
Meanwhile make the chocolate wafers. Melt the chocolate in a bowl over barely simmering water. Stir until quite smooth. Spread on a flat piece of heavy, white notepaper or light card. Put into a cold place until stiff enough to cut in square or diamond shapes.
While the chocolate is setting, make the orange-flavoured cream. Grate the rind from half an orange, add to the cream and add a pinch of castor sugar to taste. Peel and segment the oranges. Decorate the top of the mousse with orange segments and pipe on some rosettes of orange-flavoured cream. Peel the chocolate wafers off the card and use them to decorate the edges of the mousse.