Preheat the oven to 170C fan.
In a bowl cream together the sugar, butter and vanilla extract until lighter in colour and fluffy.
Add the eggs one at a time, mixing in between each one until fully combined before you add the next one.
Zest the 2 Florida Grapefruit and add to the mixture. Juice the grapefruits into a pan for later.
Sieve in self raising flour to the butter, sugar and egg mixture and fold in gently, careful not to knock any air out.
Pour the cake mix into a 2lb lined loaf tin.
Add the walnuts to the pan of grapefruit juice and warm gently for a couple of minutes, remove the walnuts from the pan and place on top of the sponge. Any excess liquid from the walnuts and grapefruit, boil down to a syrup and brush on top of the cake once it has baked.
Bake for 30 minutes or until the top of the cake is firm when pressed down gently.
Brush with the remaining syrup and serve once cooled.
