Preheat the oven to 200C/180C Fan/Gas 6. Line 6 holes of a muffin tin.
Combine the chopped rhubarb and sugar in a small bowl and set aside.
Mix together the flour, oats and baking powder.
Make a well and add in the milk, butter, oil and egg. Stir together gently with a wooden spoon.
Add the sugar/rhubarb mixture (and any liquid in the bowl) and stir together until just combined.
Scoop the mix into the muffin liners, never filling beyond 2/3 full. If you find you have too much mixture, fill another muffin liner. Optional: Scatter over some remaining oats.
Bake for 18-20 minutes, until golden brown on top. Let cool on a wire rack.
Make the drizzle by combining the instant custard and boiling water. It will be thick. Add the icing sugar. Drizzle/spoon over the muffins and serve!