Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Grease a 23x33cm / 9x13inch rectangular cake tin and line with a strip of baking parchment.
Whisk together the sifted plain flour, baking powder, bicarbonate of soda, ginger, mixed spice, caster sugar and salt in a large bowl.
Add the golden syrup, treacle, sunflower oil and boiling water to the bowl and whisk until no lumps of flour remain.
Pour the batter into the prepared tin and bake for about 25 -35 minutes, until a skewer inserted into the centre comes out clean.
Leave to cool in the tin for 20-30 minutes then carefully flip the cake out onto a wire rack and leave to cool completely.
To make the lemon glaze, place the sifted icing sugar in a bowl and gradually stir in enough lemon juice to reach a drizzling consistency. Drizzle the glaze over the cake, cut into squares and serve.