Preheat the oven to 150°C (130°C fan, gas 2) and line a 20 cm square tin.
Gently melt the butter, sugar, treacle and golden syrup in a saucepan without boiling.
Mix the oatmeal, flour, ginger, mixed spice and bicarbonate of soda in a bowl.
Pour in the warm syrup mixture, add the egg and milk and beat to a smooth batter.
Pour into the tin and bake for 45 to 55 minutes until firm at the edges and slightly soft in the middle.
Cool in the tin, wrap, and store in an airtight tin for at least 3 days before cutting.