Put the apple chunks, water and sugar into a saucepan over a lowish heat and cook until the apples are soft and mushy; this should take about 10 minutes. While cooking, stir it every minute or so to prevent sticking. Taste and add more sugar if it needs it.
Transfer the apple pulp into individual bowls or one big pie dish and allow to cool slightly. Stir the berries into the apples.
To make the crumble, rub the butter into the flour until it resembles very coarse breadcrumbs. However, if you rub it too much the crumble will not be crunchy. Combine with the sugar. Sprinkle the crumble mixture over the slightly cooled apple and bake for 15 mins for small crumbles or 30-45 mins for larger crumbles. Serve warm with whipped cream or vanilla ice cream.