Put half the sugar and half the butter and half the cinnamon in a large saucepan over low heat until both dissolve. Meanwhile, peel, core and slice the apples. Add the apples to the saucepan, give a good stir,cover the pan and cook over medium heat until apples begin to soften. Add the stoned and choped plums, stir and cook for about 10 minutes, until the mixture has the consistency of a thick compote. Taste and add more sugar or cinnamon. Put the compote in a shallow ovenproof dish Prehat the oven to 180C (350F) Stir the flour and remaining cinnamon well together, then rub in the remaining fat until the mixture resembles fine breadcrumbs. Stir in the rest of the sugar with the chopped hazelnuts. Spoon the crumble mixture over the fruit compote and bake in the oven for about 40 minute or until the top is golden, crisp and crumbly.