Mix together the dried fruit and orange zest in a bowl. Set aside. Put the yeast in a jug with 15g caster sugar. Pour over 4 tbsp warm milk and leave to foam for 5 minutes.
In a food processor, pulse the flour, 110g caster sugar, a pinch of salt, the mixed spice, cinnamon, a good grating of nutmeg and the softened butter until the mixture resembles breadcrumbs. Tip into a large bowl and make a well in the centre. Pour in the yeast mixture and most of the remaining milk. Mix to a sticky dough, adding more milk if needed.
Tip on to a floured surface and knead for 8-10 minutes, until smooth and elastic. Add the dried fruits and zest mixture and knead until combined. Put in a lightly oiled bowl, cover with oiled clingfilm and leave in a warm place to rise for 1 hour until doubled in size.
Press the dough into a rectangular shape. Divide and shape into 16 balls of equal size. Place, almost touching, on baking trays lined with baking parchment. Cover with a damp tea towel and leave in a warm place until doubled in size and joined together – this may take up to an hour, depending on the temperature.
Meanwhile, preheat the oven to 230°C/fan 210°C/gas mark 8. Beat the egg yolk with 1 tbsp milk and gently brush the buns with the mixture.
Mix together the plain flour and 1 tbsp melted butter with 4 tbsp water to form a paste. Spoon into a piping bag with a plain nozzle and pipe a cross on the top of each bun. Bake in the oven for 5 minutes, then reduce the oven temperature to 190°C/fan 170°C/gas mark 5, and bake for a further 10-15 minutes until risen and golden. Remove from the oven and cool on a wire rack.
In a small pan, gently heat the remaining sugar with 2 tbsp water to dissolve. Bring to the boil, then brush over the cooled buns. Allow to cool completely before serving.