Peel, core and slice the apples. Pick over the blackberries. Place the apple slices and the water in a saucepan and cook gently over low heat until the apples are half cooked. Add the sugar and blackberries and cook until the fruit is tender and juices have formed. Draw off the heat. Break the trifle sponge cakes into the base of a glass serving dish and spoon over the fruit and sufficient juice to soak the sponge cakes. Set aside while preparing the custard.
Custard for trifle made using a little cornflour helps to economize on the number of eggs required. The milk is thickened slightly first and then used to make the custard in the usual way. Measure the cornflour into a small basin. Add about
Crack the egg yolks into a large basin, add the sugar and stir well to mix. Stir in the hot milk mixture and blend thoroughly. Rinse out the milk saucepan and strain the custard back into the pan. Stir constantly over low heat until the custard has thickened, but do not allow to boil. Stir in the vanilla essence.
Draw off the heat and dip the base of the pan in cold water to prevent overcooking. Stir occasionally until cool and then pour over the fruit and sponge base. Leave until quite cold, then chill for several hours. Sprinkle with toasted flaked almonds and serve with single cream.