Preheat the oven to 200ºC/180ºC fan/gas mark 6. Begin by making the mincemeat. In a large food processor, add all the dried fruit and nuts until roughly chopped. Add the rest of the ingredients and whizz again until combined. Set aside until needed (this can be made up to 2 weeks ahead and stored in an airtight container in the fridge).
Unroll the pastry and cut out 20 x 8cm circles, re-rolling on a lightly floured surface as necessary. Gently push the pastry rounds into the base of the bun tins. Add a heaped spoonful of the mincemeat to fill each one.
Roll out the remaining pastry and cut out 10 star shapes and 20 holly leaves. Decorate each pie with either two holly leaves or a star shape.
Use the remaining pastry to form small balls (for the berries) for the centre of each holly mince pie. Brush the top of each pastry decoration with the egg yolk and sprinkle with Demerara Sugar.
Bake in the preheated oven for 20-25 minutes, or until the pastry is lovely and golden. Either serve warm straight from the oven or allow to completely cool on a wire rack. Store in an airtight container.