Preheat the oven to 325F or 170°C Wash, halve and remove the pit from the plums. Place plums in a shallow, ovenproof dish, sprinkle them with 50g/2oz of the sugar and 1 teasp of the cinnamon.
In a small mixing bowl combine the flour and remaining cinnamon. With a fork or clean fingertips, cut in or rub together the butter into the flour and cinnamon mixture until it resembles fine breadcrumbs. Stir in the oats and remaining sugar, spoon the mixture lightly over the plums and smooth over the top.
Bake for approximately 40 minutes, or until the crumble topping is crispy and golden brown, the plums tender.