Blend together the mixed spice, mixed peel and sultanas in a bowl and set to one side. Warm the milk to body temperature; it should feel neither warm nor cold when you dip your finger in.
Put the flour in a large bowl and rub in the butter with your fingertips. Stir in the yeast, sugar and ½ teaspoon of salt. Then mix in the eggs and the milk with one hand to form a soft, sticky dough. Mix in the dried fruit mixture then tip on to a work surface and work the dough – slide your fingers underneath then swing the dough upwards and then slap it back down, away from you.
Stretch the front of the dough towards you, then lift it back over itself in an arc, still stretching it forwards and sideway s and tucking it in around the edges. Keep repeating this sequence. Don’t add flour to the work surface at this stage. The dough will come together after a while and feel elastic. Keep on working until it comes cleanly away from the work surface; this will take about 15 minutes. Now, lightly flour the surface and form the dough into a smooth ball, folding the edges into the centre.
Alternatively make the dough in an electric mixer with a dough hook. Return the dough to the bowl, cover with a clean tea towel and leave in a warm room to rise for 1½-2 hours. It should double in size. When the dough has risen, tip it on to your work surface and divide into 16. Roll each piece into a tight ball and put on a baking tray close together. For the egg wash, lightly beat the egg with a pinch of salt and brush over each bun, keep the rest for later. Put the buns in a warm place for about 1½ hours until risen and neary doubled in size again.
Preheat the oven to 220°C, fan 200°C, gas 7. To make the cross paste, sift the flour into a bowl, add a pinch of salt, the oil and 2-3 tablespoons of cold water, a little at a time, to form a paste. Brush the buns with spare egg wash and pipe a cross on top of each bun. Bake for about 15 minutes until golden brown. Meanwhile, to make the glaze, put the sugar and 50ml of water in a saucepan. Bring to the boil and continue to boil for a few minutes until syrupy. Stir in the rum, if using. Brush over the warm buns and leave to cool on a wire rack.