Preheat the oven to 180°C, fan 160°C, gas 4. Line 3 baking sheets with greaseproof paper or grease the trays with a little butter.
Put the flour, bicarbonate of soda, ginger and cinnamon into a mixing bowl.
Cut the butter into small pieces and rub it into the dry ingredients with your fingertips until the mixture resembles breadcrumbs.
Crack the egg into a jug, add the sugar, syrup, treacle and orange zest, and mix together. Add this to the flour and stir to combine. Use your hands to squeeze the mixture together and shape into a ball.
Knead the dough on a lightly floured work surface for 1-2 minutes until smooth. Wrap in clingfilm and rest in the fridge for 10 minutes.
Sprinkle a little more flour over the work surface and roll out about a quarter of the dough to about 5mm thick, then cut out small gingerbread men and put the biscuits onto the baking sheets. Repeat with the remaining dough. Bake for 10 minutes until golden. Let them cool for a few minutes, then put them onto a wire rack to cool completely.
Mix the icing sugar with about 1 tablespoon water and use, with the currants, to decorate your gingerbread men.