Put apples into a heavy based saucepan. Add 125ml water and bring to the boil. Cover and simmer over a gentle heat for 5 mins.
Drain the apples and add to the pears and sugar. Mix well with a large wooden spoon and turn into a 9 inch ceramic flan dish.
Sift the flour into a mixing bowl, add the butter or margarine and rub in with your fingertips until the mixture resembles fine breadcrumbs. Stir in the caster sugar.
Spoon the crumble mixture evenly over the apples and pears. Bake for 35 – 40 mins, or until the crumble is golden and the fruit is cooked. Serve hot, with custard or cream.