Put the raisins, cranberries and mixed peel in a bowl, pour over the rum, and leave to soak.
Put the flour into a large bowl and stir in ½ tsp salt, yeast, sugar, nutmeg and lemon zest. Rub the butter in until the mix is like fine crumbs.
Warm the milk to hand-hot, beat the egg and mix together.
Pour the milk into the flour mix, then stir with a spatula to make a soft dough. If the mixture seems very dry and there is still bits of dry dough that are not combining then add a few drops of milk.
Lightly flour your work surface. Form the dough into a ball, then knead for about 4 minutes. Place the dough into a lightly oiled bowl and cover with a linen cloth or cotton cover. The dough will double in size in about 1 hour.
Knead the dough 2 or 3 times, then roll out to a rectangle about 37cm x 17cm.
Stir the chopped pistachios into the soaked fruits, then spread one-third of the fruit mix down the centre of the dough, leaving all the edges uncovered. Fold each side into the centre to overlap slightly. Press the edges with the rolling pin to seal, turn the dough, then repeat the rolling out and spreading of the fruit twice more. Roll out to a final rectangle that is 55cm x 16cm. Roll the marzipan into a sausage shape, so it’s slightly shorter than the length of the dough. Place it down the centre. Roll the dough over the marzipan, then turn the dough over so the join is underneath. Lift onto a buttered baking sheet, then curl the dough round a buttered 12cm round cake tin, brushing the ends with a little beaten egg and pinching them well together to seal.
Cover and leave to rise for 30-45 mins or until puffy and doubled in size.
Pre-heat the oven to 190ºC, 170ºC (fan) / gas 5. When the dough has doubled, brush with beaten egg and bake for about 25 mins until golden. Cool on the baking sheet briefly, then transfer to a wire rack.
Put the icing sugar in a small bowl and add 2 or 3 tsp water.
When the stollen has cooled, drizzle the icing over the top and scatter with flaked almonds, pistachios and cranberries.