Using a small sharp knife cut the apples into quarters, cut out the core and peel off the skin.
Thinly slice the apples and put into a 1 litre ovenproof dish.
Open the can of blackberries and tip the fruit and juice over the apples.
Trail 2 tablespoons of the syrup over the fruit.
Bake in a moderate oven (Gas No. 4 - 350°F - 180°C) for 20 minutes.
Put the other 2 tablespoons of syrup and the margarine into a small saucepan.
Put over a low heat to melt the margarine.
Remove from the heat and stir in the cornflakes with a tablespoon.
Remove the dish of fruit from the oven and spoon the cornflake mixture on the top.
Return to the oven for 25-30 minutes until the topping is crisp and golden.