substituting black treacle/molasses for brown sugar
A standard substitute for brown sugar is to take 1 Cup = 200 g of white granulated sugar, and mix-in 1 tablespoon of molasses/treacle.  There's a lot of flavor in that tablespoon.  

My own recommendation would be to either supplement your full 200 g of brown sugar with a tablespoon of treacle or consider using 200 g of granulated sugar and mix-in 2 tablespoons of treacle.

Since you are planning to use dark soft muscovado sugar (which is already flavorful) in your recipe, I think supplementing the full 200 g amount with 1 tablespoon of treacle would give your cake a terrific taste.