1 Heat the oven to 180°C (160° fan) 350°F gas 4. Line a 20cm x 30cm|8" x 12" baking tin with non-stick baking paper.
2 Put 100g|3 1/2oz of the margarine in a mixing bowl with the sugar and beat until pale and fluffy.
3 Whisk in the eggs and yoghurt, then gently fold in 200g|7oz of the flour.
4 Spoon into the tin, level the surface and arrange the plum slices on top.
5 Rub the remaining butter into the flour until crumbly and stir in the demerara sugar. Sprinkle over the cake.
6 Bake for about 40 minutes until golden and a skewer inserted into the centre comes out clean. Serve warm or at room temperature.