Heat oven to 180C. Beat butter and caster sugar together for several minutes until light and fluffy, then beat in the grapefruit zest.
Beat in the eggs one at a time.
Fold in the flour and salt. Stir in the milk.
Put the mix into a lined loaf tin and bake for 45-55 minutes until golden and a skewer will come out clean. Just before the cake is cooked mix together the grapefruit juice and granulated sugar.
Remove from the oven and prick the cake all over with a skewer. Spoon the juice/sugar mix all over the cake slowly allowing it ti soak in.
Sit patiently until it is completely cool and you can then consume it!