Break the christmas pudding into pieces and place in a flameproof dish. Mash lightly with a fork and spoon over the sherry. Cover and allow to stand for 30 minutes. Preheat oven to 180 ?C, 350 F, Gas Mark 4. Pare the zest from the oranges using a zester. Peel the oranges, removing all the pith and slice the fruit into segments. Dissolve the dark brown sugar with 1 tablespoon of orange juice in a small pan over a gentle heat until the crystals have dissolved. Increase the heat and allow to bubble until the syrup turns dark brown. Remove from the heat and add the cognac. Stir the orange segments and kumquats into the caramel. Arrange on top of the Christmas pudding. Mix the custard with half the orange zest and smooth over the fruit. Whisk the egg whites until frothy. Whisk in half the sugar until soft peaks form. Fold in the remaining sugar. Pile on top of the trifle. Bake for 25-30 minutes until pale golden. Remove and decorate with remaining orange zest. NOTES : A delicious wintery alternative to traditional trifle.