Preheat the oven to 180°C (350°F/Gas 4). Line two baking trays with baking paper.
Sift the flour, bicarbonate of soda, ginger and mixed spice into a large bowl. Add the butter and sugar and rub into the flour with your fingertips until the mixture resembles fine breadcrumbs. Pour the boiling water into a small heatproof bowl, add the golden syrup and stir until dissolved. Add to the flour and mix to a soft dough with a flat-bladed knife.
Roll into balls using 2heaped teaspoons of mixture at a time. Place on the trays, allowing room for spreading, and flatten out slightly with your fingertips. Bake for 15 minutes, or until the biscuits are well coloured and firm. Repeat with the remaining mixture. Cool on the trays for 10 minutes, then transfer to a wire rack to cool completely. Store in an airtight container.