To prepare the cranberry swirl, combine the frozen fruit, zest, water and salt in a sauce pot over medium heat. Simmer for 20-30 minutes, or until the liquid has reduced and the contents have thickened. Reserve.
For the bread batter, sift the flour, salt, baking powder and baking soda into a bowl and set aside. In a separate bowl, beat the butter with a hand mixer until soft. Add the sugar and continue to beat until the mixture is pale yellow and fluffy. Add the eggs, one at a time, beating between each addition. Add the vanilla and the yoghurt. Beat until smooth. Stir in the flour mixture until fully incorporated.
Pour the contents into a greased and floured 9 inch loaf pan. Spoon the cranberry swirl over the top of the batter in one even layer. Gently run a spoon through the mixture, being sure not to over mix. Bake at 325 degrees F / 162 degrees C for 10 minutes. Reduce the heat to 300 degrees F /148 degrees C. and continue baking for another 20 minutes, or until a cake tester inserted into the center comes out clean.