Preheat the oven to 200°C
Line a 12 hole muffin pan with paper cases
Sift the flour into a bowl
Add the soft brown sugar
Whisk the egg, milk and butter together
In a small bowl, mix the rhubarb, caster sugar and water together
Make the crumble topping by putting all the ingredients, except the Demerara sugar, into a bowl and rub the butter in with your fingertips until the mixture resembles rough breadcrumbs. Add the sugar.
Add the egg mixture and the rhubarb mixture to the flour and fold it all together. Don’t over mix, it is supposed to be lumpy
Divide the muffin mixture equally between the paper cases
Top each muffin with a generous amount of crumble topping and press it on very gently
Bake for about 20 minutes, leave the muffins in the tin for 5 minutes before transferring to a wire rack to cool.