Preheat the oven 180C/160C Fan/Gas 4. Line a 30x23x4cm/12x9x1½in traybake tin with baking paper.
Mix the flour, bicarbonate of soda, ginger and mixed spice in a large bowl.
Gently heat the butter, treacle, syrup and sugar together in a small saucepan until the butter has just melted. Set aside to cool slightly.
Pour the butter mixture into the dry ingredients, add the egg and beat with a wooden spoon. Gradually pour in the milk and beat until smooth.
Pour into the tin and bake for about 35 minutes, or until dark golden and springy to the touch. Leave to cool in the tin for 10 minutes, then remove from the tin and place on a wire rack to cool completely.
Tightly wrapped, this will keep for up to a week. It also freezes well.