On my house I have a problem with mould in the window reveals. This is because it is a cold spot in the construction of the house. The walls here are effectively thinner than the rest as the window only sits back a few inches from the outside of the house. With my windows also not being the best it is a perfect haven for condensation and hence mould. Cool and wet is not the best combination in a room.
So what to do about it?
Well basically it just needs some additional insulation to keep it warmer. By doing this it will stop the water in the air from condensing on the walls and hence stop the perfect conditions for mould.
There are a few products that can help here. I will avoid using the phenolic backed plaster boards as I have developed a loathing for them. They are a legacy waste (what do you do with them at the end of their life?) and also can trap moisture behind if not installed perfectly.
Given that your walls will no doubt already have paint on I think that the easiest solution is to use InsOwall insulating plaster. It is very sticky and so you can prep the walls easily just by scratching back some areas to the plaster and then just with some wood to give an edge you can easily apply the plaster (up to 40mm thick) into the reveals. This can then be plastered over to give a really good finish with lime plaster, clay plaster or gypsum (if you must). It is possible to get a final finish with the InsOwall, but in reveals this can be a little tricky to the DIYer.
Other options would be:
Hemp / Lime plaster for those homes where good breathability is essential (InsOwall is a semi-breathable material)
Calsitherm, again where good breathability is required, but also some additional thickness and strength
You could also use Aerogel, but it is expensive and getting it in small enough quantities would be an issue.