Preheat the oven to 140C/275F/Gas 1. Grease a 20 x 30cm/8 x 12in cake tin and line with greaseproof paper.
Sieve the flour, salt, ginger, nutmeg and mixed spice into a large bowl. Mix in the oats.
Place the golden syrup, treacle, butter and sugar into a small saucepan and heat gently until melted and well combined. Stir this mixture into the bowl with the dry ingredients.
Add the beaten egg and milk to the bowl and mix to make a soft, almost pouring, consistency. Pour the mixture into the cake tin.
Transfer to the oven and bake for 1-1¼ hours, or until firm in the centre. Remove from the oven and leave in the tin for 5-10 minutes before turning out.
Meanwhile, for the roasted rhubarb, place the rhubarb pieces onto a baking sheet and sprinkle with sugar, to taste, and a splash of water. Transfer to the oven and roast for 10-15 minutes, until softened.
For the syrup, place all the syrup ingredients into a small pan and whisk together over a low heat until well combined.
To serve, cut the warm parkin into wedges, top with the rhubarb and drizzle over the syrup. Serve with a scoop of ice cream and a sprig of mint, to garnish.