Preheat the oven to 150 degrees C. Grease and line a 30cm x 21cm baking tin with baking paper.
In a saucepan over a low heat combine the butter, treacle, golden syrup and sugar then stir until melted together. Remove from the heat and allow to cool a little.
In a large mixing bowl, combine the flour, bicarb and oatmeal then mix. Gradually stir in the butter mixture followed by the egg then the milk to form a thick mixture. Pour into the prepared baking tin.
Bake in the preheated oven for 90 minutes, until a skewer inserted into the centre comes out clean. During the first hour of cooking resist the urge to open the oven door to check on the parkin as this may result in the middle sinking. Remove from the oven and allow to cool in the tin, ideally leaving to rest overnight. Cut into squares and serve.
You can make parkin several days in advance and store at room temperature in a sealed container, it is said to improve with a few days resting.