Grease a large loaf tin with a base measurement of about 25 x 10 cm (10 x 4 inches). Put the flour, yeast, mixed spice and sugar in a bowl. Melt 25 g (1 oz) of the butter, mix with the milk and add to the bowl. Mix with a round-bladed knife to make a soft but not sticky dough. Turn out onto a lightly floured surface and knead for 10 minutes until smooth and elastic. (Alternatively, use a freestanding mixer with a dough hook and knead for 5 minutes.) Place in a lightly oiled bowl, cover with clingfilm and leave to rise in a warm place for about 1½ hours or until doubled in size.
Turn the dough out onto a floured surface and knead in the sultanas, almonds and candied peel. Cover loosely with a tea towel and leave to rest for 10 minutes. Roll out the dough on a floured surface to a 25 x 20 cm (10 x 8 inch) rectangle. Roll the marzipan under the palms of your hands to form a log shape about 23 cm (9 inches) long and flatten to about 5 mm (¼ inch) thick. Lay the marzipan down the length of the dough, slightly to one side, and fold the rest of the dough over it. Transfer to the tin and press down gently.
Cover loosely with oiled clingfilm and leave to rise in a warm place for about 30 minutes until slightly risen. Remove the clingfilm. Bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 25 minutes until risen and golden. Leave for 5 minutes, then turn out of the tin, place on a wire rack, cover with a sheet of foil and place a couple of food cans or pack of icing sugar on top to keep the stollen compact, while cooling on the wire rack. Melt the remaining butter and brush over the bread. Dust generously with icing sugar.