Preheat your oven to 160C, 140 fan, 325 F, Gas mark 3.
Line a 450g loaf tin.
Cream the butter/margarine and sugar until light and fluffy, then beat in the lemon rind and egg yolk. Stir in the flour, baking powder and oats, then mix in the tonic water and stir until you have a smooth mixture. Beat the egg white until stiff and fold in gently. The egg white adds lightness to the texture.
Pur into the loaf tin and bake for around 35 minutes, until risen and firm and the point of a skewer inserted in the middle of the cake comes out cleanly. Drizzle with the gin mix while warm and leave to cool in the tin.
Once the cake is cold, make a pouring water ice with icing sugar and gin and/or lemon juice if preferred and drizzle over the cake.