First make the bread dough. Warm the milk in a saucepan until you can see steam coming from the surface, but don’t let it boil. Add the butter and set aside. Tip the flour, yeast, sugar and ½ tsp salt into a large bowl. When the milk has cooled to hand temperature and the butter has melted, pour the liquid into the bowl. Mix with a wooden spoon, then tip out onto your work surface and bring together with your hands. Knead the dough for 10 mins or until soft. Put it in a clean, lightly oiled bowl and cover with a sheet of oiled cling film. Leave somewhere warm for about 2 hrs until doubled in size.
Place the currants, mixed peel, cinnamon, mixed spice, zests and juice into a saucepan. Boil hard for a few mins until the fruit has absorbed the liquid and become nice and plump. Chill until cold, then mix with the pistachios, butter and sugar.
When the dough is ready, tip out of the bowl and knead briefly to knock out the air bubbles. Roll out to a 45 x 28cm rectangle. Spread the buttery spiced fruit mixture over the surface of the dough. From one of the long edges, roll the dough to a tight cylinder, then bring the 2 ends to meet, creating a ring. Place on a lightly floured baking sheet with the join underneath.
Using a sharp knife, cut incisions around the outside of the dough, about 3cm apart, in a sunburst pattern, going quite close to the centre, but not all the way through. Working your way around the outside, pull each piece of cut dough out a little, then push it down on one side, so that the cut edge is exposed and they’re on a curve to create a fan shape around the edge of the wreath. When you’re happy with the shape, re-cover with oiled cling film and leave to prove for about 45 mins until doubled in size. Heat oven to 200C/180C fan/gas 6.
Uncover the bread and brush all over with beaten egg. Bake for 25-30 mins until golden and hollow sounding when tapped on the base. Mix a little icing sugar with enough water to make a smooth runny icing. Whilst the bread is still warm, brush the icing over the surface with a pastry brush to make a sugary crust. Best eaten within 3 days.