Put the blackberries, apples, measured water and sugar in a saucepan, then cover and simmer for 5 minutes or until the fruit has softened. Leave the mixture to cool slightly.
Break the trifle sponges into chunks and arrange in an even layer in the base of a 1.2 litre (2 pint) ovenproof pie or soufflé dish and drizzle with the sherry. Spoon the poached fruit and syrup over the top, then cover with custard.
Whisk the egg whites in a large, dry bowl until stiffly peaking, then gradually whisk in the sugar, a teaspoonful at a time, until the meringue is stiff and glossy (see
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 15–20 minutes until heated through and the meringue is golden. Serve immediately.