In a small bowl, stir together the mixed spice, baking powder, salt, buckwheat flour, ground almonds and currants. Pour these dry ingredients into the medium bowl and stir well to combine. Scrape the mixture into the cake tin, spreading it out evenly in the pan, and bake for 40 minutes. The top will look dry and dark golden. Run a dull knife around the edge of the cake and turn it out onto a wire rack. Remove the paper from the cake and leave to cool.