Simmer the damsons, apples, water and first lot of sugar together in a small saucepan for around 10 minutes or until just softened.
Put the fruit into an oven proof dish and get the oven on at 190C.
For the crumble, rub the flour into the butter until you've got lumps, breadcrumbs or whatever level of crumbliness suits you. Then stir through the light muscovado sugar.
Spoon the crumble mixture onto the damsons and spread out evenly over the top. Scatter the oats over the crumble.
Place in the oven and wait 45 minutes or until lusciously golden brown.
Notes
If you don't have oats for the top, don't worry, you can leave them off or sprinkle a little extra sugar on top instead.