Steep the currants in the cider until plumped. Rub the butter into the flour until the mixture resembles fine crumbs. Sift together the nutmeg, cream of tartar, soda and sugar, and add to the mixture. Mix the eggs into the buttermilk with the flavouring. Make a well in the dry ingredients. Mix in the cider and the buttermilk alternately, adding about one-third each time. Mix thoroughly to blend. Place the mixture in an eight-inch cake tin, lined and greased, and bake at 325°F, 170°C, gas mark 3 for two hours or until the cake is springy and brown. Cool well before cutting.