Preheat the oven to 180°C/350F/Gas 4. Grease and line a 2lb loaf tin.
Add 1.5 tablespoons of freshly squeezed orange juice to the milk and leave to stand for about 5 minutes until curdled and lumpy.
Sift the flour, baking powder and bicarbonate of soda together into a large bowl.
Add the caster sugar, orange zest and salt and stir to combine.
Add the oil and vanilla to the milk mixture and pour into the flour mixture. Fold in, gently, with a metal spoon until combined. Don't worry if there are a few small lumps.
Mix the cinnamon and brown sugar together in a separate small bowl. Add 4 tbsps of the cake mixture and stir to combine.
Spoon half the remaining cake mixture into your prepared tin, top with the cinnamon and sugar mixture and gently level the surface. Top with the remaining cake mixture and level gently again.
Use a skewer or fork to gently swirl the mixture.
Bake for 30 – 40 minutes until golden brown. Test if it’s cooked by pushing a skewer into the centre, if it comes out clean it is ready.
Leave to cool for 10 minutes before turning out on to a wire rack to cool completely.
Once the cake is completely cool make the icing. Sift the icing sugar into a small bowl and add orange juice a little at a time until you have a spooning consistency. You won't need much liquid. If you add too much just add a bit more icing sugar until you have the right consistency.
Place the cake and its cooling rack over a baking tray and drizzle the cake with the icing. Sprinkle with a little orange zest and serve.