For the buns, sieve the flour, salt and ground mixed spice into a large mixing bowl, then rub in the butter using your fingertips. Make a well in the centre of the mixture, then add the sugar and lemon zest and yeast.
Beat the egg and add to the flour with the tepid milk. Mix together to a form a soft, pliable dough.
Turn out the dough onto a lightly floured work surface. Carefully work the mixed dried fruit into the dough until well combined. Knead lightly for 5 minutes, or until smooth and elastic.
Grease a large, warm mixing bowl with butter. Shape the dough into a ball and place it into the prepared bowl, then cover with a clean tea towel and set aside in a warm place for one hour to prove.
Turn out the proved dough onto a lightly floured work surface and knock back the dough. Shape it into a ball again and return it to the bowl, then cover again with the tea towel and set aside for a further 30 minutes to rise.
Turn out the dough onto a lightly floured work surface and divide it into 12 equal pieces. Roll each piece into a ball, then flatten slightly into a bun shape using the palms of your hands. Cover the buns again with the tea towel and set aside to rest for 5-10 minutes.
Grease a baking tray with butter and transfer the buns to the tray. Wrap the tray with the buns on it loosely in greaseproof paper, then place inside a large polythene bag. Tie the end of the bag tightly so that no air can get in and set aside in a warm place for a further 40 minutes to rise.
Preheat the oven to 240C/475F/Gas 8.
Meanwhile, for the topping, mix the plain flour to a smooth paste with 2 tablespoons of cold water.
When the buns have risen, remove the polythene bag and the greaseproof paper. Spoon the flour mixture into a piping bag and pipe a cross on each bun.
Transfer the buns to the oven and bake for 8-12 minutes, or until pale golden-brown. As soon as you remove the buns from the oven, brush them with the hot golden syrup, then set aside to cool on a wire rack.