Cook the plums with the vegan margarine in a saucepan over a gentle heat for 3 minutes. Add the apples and demerara sugar and cook, stirring occasionally, for a further 2–3 minutes.
Remove the pan from the heat, add the bananas and spices and toss gently to lightly coat all the fruit in the sugar and spice. Divide the mixture between 4 x 250 ml (8 fl oz) gratin dishes.
Put the flour for the crumble in a bowl, add the spread and rub in with the fingertips until the mixture resembles fine breadcrumbs. Stir in the sugar and oats. Spoon evenly over the top of the fruit in each dish, place the dishes on a baking sheet and then bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes until the crumble is golden and the fruit is bubbling. Serve with natural soya yogurt if liked.