Preheat the oven to 180°C, fan 160°C, gas 4. Put the apples, orange zest and juice and 60g of the caster sugar in a pan and simmer gently for 15 minutes, until the apples begin to soften. Transfer the cooked apples to a 1.25 litre ovenproof dish.
Put the remaining sugar, the flour, oats, butter and cinnamon in a food processor and pulse until the mixture resembles coarse breadcrumbs.
Scatter the crumble mixture over the fruit and bake for 30-40 minutes, until golden. Serve warm with the natural yogurt.