Preheat the oven to 160c fan
Grease and line three 7 or 8 inch layer cake tins with baking paper (this will make three thin layers)
In the bowl of a stand mixer fitted with a whisk attachment whip the butter and sugar until pale and fluffy,
Scrape around the bowl with a spatula and add the first egg and whip for about 3 minutes, then scrape the bowl and add the next egg. whip again for 3 minutes, then scrape the bowl and add the next egg. continue this process with the last two eggs
Sift in the flour, baking powder and salt and fold in by hand with a spatula.
Now fold in the zests, peel and juice, Add a tbsp of milk at a time to loosen the batter.
Pour evenly amongst the tins and level out.
Bake for 20 to 22 minutes or until a knife inserted in the centre comes out clean.
Leave to cool in the tins for 20 minutes, then carefully remove from the tins and peel off the paper and place on a cooling rack until cool.