Preheat oven to 180 degrees C. Spray a loaf tin with cooking spray or line with baking paper.
Sprinkle chopped plums with 1 tablespoon of flour in a bowl and toss to coat. Set aside. In a large bowl beat the margarine, white sugar and vanilla essence with an electric mixer until light and fluffy. Beat in the eggs. In another bowl, sift together 1 1/2 cups of flour with salt and bicarb soda. Stir the dry ingredients into the egg mixture, alternating with yoghurt until the mixture forms a smooth batter. Lightly stir in the flour-coated plums and pour the batter into the prepared loaf tin. Sprinkle the top of the batter with brown sugar.
Bake in the preheated oven until a toothpick inserted into the centre of the loaf comes out clean, 45 to 50 minutes. Remove from oven and let cool in the pan 10 to 15 minutes before removing from pan for slicing.