Preheat oven to 170 C / Gas 3. Grease and flour two 23cm tins. Sieve together the flour, baking powder and cream of tartar. Set aside.
In a medium bowl, beat together the 8 egg yolks and 400g sugar until thick and pale. Stir in the 2 teaspoons lemon zest and 2 tablespoons lemon juice. In a large glass or metal mixing bowl, beat egg whites and salt until soft peaks form. Fold whites into the egg yolk mixture alternately with the flour mixture. Spread evenly into the prepared tins.
Bake for 25 to 30 minutes in the preheated oven or until a skewer inserted into the cake comes out clean. Let layers cool in the tins for 15 minutes before inverting onto wire racks to cool completely. Using a long serrated knife, slice the layers in half horizontally.
Meanwhile, make the filling. In the top of a double boiler, combine the 275g sugar, 4 egg yolks, 2 1/2 teaspoons lemon zest and 75ml lemon juice. Cook over high heat, stirring constantly, until the sugar is dissolved and mixture thickens. Remove from heat and stir in the butter. Cool to room temperature before filling cake.
For the icing: In a medium bowl, cream the 75g butter until light and fluffy. Gradually add the icing sugar and mix in the orange zest, orange juice, lemon zest and lemon juice. Finally, stir in coconut. Ice the outside of the filled cake.