Preheat the oven to 180 C / Gas 4. Butter and flour two 20cm round cake tins.
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 2 tablespoons lemon juice and 2 tablespoons zest. Pour cake mixture into prepared cake tins.
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.
Starting with a clean and cold metallic or glass bowl, beat the cream until stiff peaks form. Fold in 2 tablespoons lemon juice and 1 tablespoon lemon zest.
Spread lemon curd on top of one cake layer. Place second cake layer upside-down on top of lemon curd. Top cake with whipped cream.