Don't preheat oven. Grease and flour a ring tin. Sift together the plain flour and self-raising flour; set aside.
In a large bowl, cream together the margarine and sugar until light and fluffy. Beat in the eggs one at a time. Alternately beat in the flour mixture and the milk, mixing just until incorporated. Finally, stir in the lemon essence and vanilla.
Pour batter into ring tin. Place cake in cool oven and set oven to 180 degrees C. Bake for 1 hour and 15 minutes, or until toothpick inserted in centre comes out clean.