Heat the oven to 180°C (160° fan) 350°F gas 4. Grease 4 individual pudding basins or baking dishes.
For the fruit filling: put the sugar, apple and pear cubes into a pan. Add just enough water to just cover the mixture. Heat gently, stirring now and again until the sugar has dissolved, then turn up the heat and bring to a boil. Turn down the heat and simmer gently for 4-5 minutes, until the fruit has softened.
For the crumble: put the butter, flour, sugar and salt into a mixing bowl and rub together with your fingertips until the mixture resembles breadcrumbs. Stir in the porridge oats until well combined.
Sprinkle the fruit mixture with cinnamon and spoon into the pudding basins.
Top with the crumble mixture and sprinkle with demerara sugar.
Cook for about 10-15 minutes, until the crumble is golden brown.