Preheat the oven to 230°C. Halve the plums and remove the stones. If the plums are large, cut into quarters.
Put the plums, the caster sugar and 2–3 tablespoons water into a pot. Cover and poach over moderate heat for 8–10 minutes, stirring occasionally, until the plums begin to soften. Transfer the plums to an ovenproof dish about 20 cm in diameter and 5 cm deep.
Meanwhile, melt the butter in a frying pan, then stir in the mixed spice, rolled oats, brown sugar and chopped nuts.
Spoon the mixture evenly over the top of the plums. Put the dish into the oven and bake for 12–15 minutes, checking regularly, until the oats are golden brown.
Serve the crumble straight from the cdish, hot or warm, with whipped cream or ice cream.