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Preheat the oven to 180°C/Fan 160°C/Gas Mark 4.
Cut ½ the grapefruit or one of the lemons into thick wedges. Zest and juice the remaining fruit and put to one side. In a large mixing bowl, combine the self-raising flour, citrus zest and 175g caster sugar. Add the butter, eggs, 50ml gin, 2 tbsp citrus juice and mix with an electric whisk until smooth.
Pour the batter into a lined loaf tin and bake for 35 minutes until golden and springy to touch. Remove from the oven but leave in the tin.
While the loaf is cooling, pour the remaining citrus juice, tonic and 75g sugar into the saucepan and heat on a medium-high heat for 5-8 minutes until the sugar has completely dissolved. Allow to cool slightly before stirring in the remaining gin. Set aside 3 tbsp syrup for the glaze.
Poke the loaf all over with a fork and drizzle with syrup from the saucepan. You can do this while the loaf is still warm.
Once the loaf is completely cool, remove from the tin. Combine the remaining syrup with the icing sugar to make a glaze and pour over the top of the loaf, slightly pushing it down the sides with the end of a spoon to create a drip effect. Decorate with the citrus slices and serve.