Place the flour in the bowl of a stand mixer fitted with a dough hook (or dough roller if you happen to have an Ankarsrum one), add the yeast to one side and the salt, sugar, mixed spice, cinnamon, nutmeg and allspice to the other. Stir to combine.
Add the milk, water, melted butter, treacle and vanilla and mix on a low speed until it forms a rough, sticky dough.
Turn the speed up to medium and continue to mix for about 10 minutes until the dough is smooth and elastic and pulls away from the sides of the bowl cleanly. You should be able to stretch the dough very thinly before it tears (windowpane test).
Shape the dough into a ball, place in a large, lightly oiled bowl, cover and set aside to rise in a warm spot until doubled in size, about 2-3 hours.
Punch down the risen dough, scatter over the dried fruit and knead until evenly distributed.
Shape the dough into a neat ball, making sure that there is no fruit on the outside or it will burn.
Place on a baking sheet lined with greaseproof paper, loosley cover and set aside to rise in a warm spot until almost doubled in size, about 1 ½ - 3 hours. It is ready to bake when the dough is puffy and springs back slowly, leaving a small indentation, if you gently poke it with a finger. If it springs back quickly and fills in completely then it needs to rise for longer.
When the dough is almost ready, preheat the oven to 190°C/170°C fan/375°F/gas mark 5.
Gently brush the dough with milk then bake for 50 minutes. Cover with tin foil towards the end of baking if it starts to get too dark. The internal temperature should reach at least 90°C/194°F on a probe thermometer when it is ready.
While the bread is baking, mix the sugar and boiling water together to make the glaze.
When the bun is baked, remove it from the oven and brush all over with the sugar glaze. Keep going until you have used all of the glaze.
Leave to cool on the tray for 5 minutes then slide it onto a wire rack and leave to cool completely before slicing.