Preheat the oven to 140°C.
Grease and line a 20 x 30cm cake tin.
Sift the flour, salt, ginger, nutmeg, and mixed spice into a bowl and mix in the oats.
Melt the golden syrup, treacle, butter, and brown sugar in a saucepan over low heat.
Stir the melted mixture into the dry ingredients.
Mix in the beaten eggs and milk to achieve a soft consistency.
Pour the batter into the tin and bake for 60 to 75 minutes until firm.
Cool the parkin in the tin for 10 minutes before turning out.
Place rhubarb on a baking sheet, sprinkle with sugar and water, and roast for 15 minutes.
Whisk syrup ingredients in a pan over low heat until combined.
Serve parkin wedges topped with rhubarb, syrup, ice cream, and mint.