Preheat the oven to 180 C / Gas 4. 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 115g butter, then add three eggs; continue processing until fluffy. Stir into flour in three parts, alternating with buttermilk. Stir in in lemon juice and zest. Pour cake mixture into 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.
Warm marmalade with 2 tablespoons butter and icing sugar. Stir until icing sugar melts. Pour over warm cake, and set aside to cool completely.