Combine all of the ingredients for the mincemeat in a large bowl and mix thoroughly. Cover with cling film and leave for 24 hours.
For the pastry, sift the flour into a bowl, add the softened butter, icing sugar and orange zest and gently incorporate with your hands until the mixture resembles rough breadcrumbs. Mix in the egg yolks and then add 2-3 tablespoons of water to help bring it together. Squeeze the pastry together gently until you have a soft ball of pastry. Flatten to a disc, wrap in cling film and refrigerate for 30 minutes.
Preheat the oven to 180C/350F/Gas 4.
Grease a bun tin or muffin tin, depending on how deep you like your mince pies. Unwrap the pastry and roll out on a lightly floured work surface to the thickness of a one pound coin. Cut out 12 discs of pastry with a pastry cutter slightly bigger than the size of the bun tin holes. Press a round of pastry into the bottom of each hole. Fill each pastry case with mincemeat and then cut a further 12 rounds of pastry (this time one size smaller). Brush the rims of the pastry cases with a little beaten egg and press the lids on, pressing the edges with the ends of a fork to seal shut.
Brush the lids with beaten egg and then use a small knife or skewer to make a slit in the top of each pie. Sprinkle with demerara sugar and then transfer the tray to the fridge for 30 minutes.
Bake the mince pies for 20-25 minutes, or until golden-brown and crisp. Transfer to a wire rack to cool and then turn the mince pies out and serve. Any leftover mincemeat should be spooned into sterilised jars and sealed.