Preheat the oven to 180 C / Gas 4. Slice the plums, removing the stones and put them in a bowl.
Add the caster sugar and rose water and toss until plums are well coated in sugar. Set aside for 10 minutes.
Toss the dark brown soft sugar, flour and coconut together in a bowl. Rub the butter in with your fingertips until the mixture resembles breadcrumbs. Set aside.
Stir the plums again to ensure they are well coated, then divide amongst six 150ml ovenproof bowls, piling the plums above the tops of the bowls (they will cook down quite a bit).
Top the plums with the coconut crumble mixture, then set the bowls on a baking tray. Bake for 30 minutes or until the crumble is golden and the juices are bubbling.
Serve warm or at room temperature with whipped cream or ice cream.