Condensation, mould and musty smell

[EMF]

I have recently purchased a 1900s ground floor flat, solid stone build. I have a large cupboard that has 2 walls on to the common hallway and there is an issue with condensation, mould and a musty smell. I'm planning on converting this cupboard in to a wardrobe an want to solve the condensation and associated issues. I'm thinking of a small extractor fan, some low and high vents and also using an electric underfloor heating mat. I'd appreciate any thoughts on this.

 

[Cubist]

A photo or two would be helpful and some understanding of where you believe the condensation is forming and also where the mould outbreaks are occurring may be informative too?