Grease a 7" cake tin with oil and sprinkle a pinch of flour over the tin to create a thin layer on the grease. This will help your cake to come out easily when it's baked. Preheat the oven to 180°C (gas mark 5).
In a large bowl beat the margarine with the sugar until light and fluffy with a folk or whisk. Add the zest and whisk well.
In a separate bowl sift together the flour, cornmeal, baking powder ans salt.
Add some of the flour mixture to the margarine mixture and beat with a spoon, then add some of the soya yogurt and mix well.
Add the rest of the flour and yogurt and combine well but DO NOT OVER MIX!! The batter will be very thick so you will need to spoon it to put in the cake tin rather than pouring it.
Bake for 30 minutes until a toothpick inserted into the cake comes out clean and golden on top. Let it cool for 10 minutes before removing from the tin and cool further on a wire rack.