Place the flour into a large mixing bowl, add the salt to one side and the yeast to the other side.
Warm the milk and butter in a small saucepan until the butter is melted and the mixture is lukewarm. Pour into the flour mixture, add the eggs and stir thoroughly until the contents of the bowl come together as a soft dough (the dough will be sticky).
Tip the dough out onto a lightly floured work surface and knead well for 5 minutes, or until the dough is smooth and elastic. Alternatively this can be done in a freestanding mixer using a dough hook.
Place the dough in an oiled bowl and leave to rise, covered with a damp tea towel, for 1 hour or until doubled in size. This is best done in a warm room.
For the filling, mix the mincemeat with the apple, pear, orange zest, pistachio nuts, candied peel and cinnamon.
When risen, tip the dough out onto a lightly floured work surface. Roll out into a rectangle about 50x45cm/20x18in. Tack down the long side of the dough rectangle nearest to you by pressing it down onto the work surface with your thumb. Brush all over with the melted butter. Then spread the mincemeat mixture over the dough leaving a 2cm/¾in border.
Roll the opposite long side of the dough towards you quite tightly, until the roll is complete and tight. Trim the ends to neaten. Using a sharp knife, cut it into 15 rounds about 4cm/1½in thick.
Line a very large baking tray (or use the grill tray from your oven) with baking parchment. Arrange the rolls on the prepared tray, cut side up, in a Christmas tree shape, with 1 on top, 2 underneath, 3 in the third row, 4 in the fourth row, 5 in the fifth row. Use some of the trimmings to make a trunk. You want them to be close enough so that their sides will touch when the dough has risen. They can then be pulled apart and you get a lovely soft edge.
Cover loosely with a clean tea towel and let the dough rise for 30-45 minutes.
Preheat oven to 190C/170C Fan/Gas 5.
When the buns have risen, put them in the oven and bake for 20-25 minutes or until golden-brown. Check after 15 minutes or so and cover the buns with foil if they are browning too quickly.
Remove the buns from the oven and let them cool slightly before transferring them to a cooling rack.
To finish, melt the jam in a small saucepan with a splash of water until smooth. Brush the jam over the buns to glaze and allow to cool completely.
Mix the icing sugar, orange zest and 2 tablespoons water together. Transfer to a disposable piping bag and snip off a tiny tip. Pipe garlands on the buns to resemble a decorated Christmas tree. Garnish with glacé cherries, candied peel and chopped pistachio nibs.