Preheat the oven to 200 C, 180 Fan, Gas 6. Place the blackberries, elderberry jelly, sugar and ground cinnamon in a pie dish and mix together.
Place the flour and hazelnuts in a food processor. Whizz until the nuts form fine crumbs then add the diced butter. Briefly process for a minute or two until the butter forms fine breadcrumbs. Remove to a bowl and add the sugar.
Cover the fruit with the crumble mixture and press down lightly but evenly. Place in the centre of the oven and bake for 25 minutes, then reduce the heat to 190 C, 170 Fan Gas 5 and cook for 15 minutes more.
Serve hot or warm with plenty of thick cream or custard.