Preheat the oven to 170?C/Fan oven 150C/325F/Gas Mark 3. Line a 23cm round springform tin with baking paper.
You could get away with a 20 cm tin but make sure that the baking paper comes up a good 2cm above the height of the tin.
Sieve the flour into a bowl and rub in the butter until the mixture resembles breadcrumbs. If you prefer, you can do this in a food processor.
Stir in the sugar, currants, spice and cream of tartar. Dissolve the bicarbonate of soda in a little of the buttermilk then add to the mix, stirring well.
Stir in the remaining buttermilk. Turn the mixture into the prepared tin and level the surface.
If using, sprinkle the Demerara over the top of the batter.
Bake for approximately 1 hour 10 minutes or until a skewer inserted into the middle of the cake comes out clean.
Stand the cake, in its tin, on a wire rack until the tin is cool enough to safely handle and remove.
Leave the cake to cool completely on the wire rack.
Bask in the glory of the wonderful thing you have made.
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