Preheat the oven to 180c fan and line loaf tin with greaseproof paper. In a large mixing bowl, stir together the flour, baking powder, caster sugar and zest.
In a separate bowl, combine the oil and water.
Pour the wet mixture into the dry and mix with a wooden spoon until combined. Pour the cake batter evenly into the lined tin.
Place the tin in the middle of the oven and bake for 30-35 minutes or until golden brown. You’ll know when it’s down when it is spring to the touch.