Cake: Preheat the oven to 180 degrees C. Butter and flour a 25cm round cake tin.
Sift together flour, baking powder and bicarb into a large bowl.
In a food processor, process sugar with the butter, then add three eggs, while continuing to process until fluffy. Stir into flour in three parts, alternating with buttermilk. Stir in in lemon juice and zest. Pour cake mixture into the prepared cake tin.
Bake for 30 to 40 minutes, or until a skewer inserted into the centre comes out clean. Remove from tin, and set aside to cool.
Glaze: Warm the marmalade with the butter and icing sugar. Stir until icing sugar melts. Pour over warm cake and set aside to cool completely.