Pour 2 cups boiling water on dried currants and soak for 15 minutes, then drain. Pour remaining water over saffron and steep until cool.
Cream butter and sugar until well blended, about 3 minutes. Add eggs one at a time, blending after each. Add lemon zest and juice.
Combine dry ingredients in a bowl and add to the sugar-butter mixture, alternately with the saffron water, beating until just blended. Stir in raisins and drained currants. Pour into a greased and floured loaf pan. Bake 40 - 50 minutes until top is golden brown and a toothpick inserted into the center comes out clean. Tip out onto a rack to cool.