Heat the oven to 180`c and line a rectangular baking tin (approx 27cm x 20cm) with parchment paper. Peel, core and thinly slice the apples then squeeze the lemon juice over. Set to one side. Place the butter, caster sugar, eggs, vanilla, flour and baking powder into a large bowl and mix well until smooth. Spread half the mixture into the prepared tin. Arrange half the apples over the top of the mixture followed by half of the cinnamon and the Christmas mincemeat. Carefully spread over the remaining cake mixture followed by the remaining apple slices. Sprinkle over the remaining cinnamon, demerara sugar and the walnut pieces. Bake in the oven for 45-50 minutes until golden and springy to the touch. Meanwhile, mix together the icing sugar with a little water to make a glace` icing and as soon as the cake comes out of the oven drizzle it all over. Serve the apple cake with a scoop of vanilla ice-cream.