Put the plums, caster sugar and orange juice in a saucepan and cook for 3 minutes or until starting to soften, then spoon into an ovenproof dish. Melt the butter and mix together with the remaining ingredients. Crumble the mixture over the plums, breaking up any large clumps.
Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 20–25 minutes until the topping is crisp. Serve with plenty of custard.