Lightly grease a 23x33cm/9x13in Swiss roll tin. In a bowl rub the butter into the flour using your fingertips. Stir in the caster sugar and orange zest. Pour in the milk and mix using a flat knife blade until it is fully combined with the dry ingredients (add a little more milk if required).
Tip the mixture into the prepared tin and smooth the surface.
For the topping, place the plums on top, cut side up, and put a small dot of butter on top of each plum.
In a bowl, mix the demerara sugar and cinnamon. Sprinkle over the plums and finish with a drizzle of orange juice.
Bake for approximately 20 minutes, or until the base is golden and plums are bubbling and juicy.