Black mould on outside walls

[max330]

6 months ago I purchased a property. The top two bedrooms had black mould on the outside walls. The place had been empty for a while. I killed the mould and re-painted the walls.
But I’m worried that the mould will be back again in winter. Pretty sure the walls are a solid brick construction. Not sure if it’s a pointing issue, or because of the wall construction, or some other reason.
These walls also had lining paper on, so not sure if that contributed to the mould growth.

One of the bedrooms in particular also had very damp walls. I checked levels with a meter and some places were 50%.

I would check the pointing, but these rooms are on the third story. If the pointing is okay, I was thinking about painting a sealer on to the outside of the brickwork.
I then read a helpful article from Emperor Paint which says that Masonry Cream is a much better product.

Has anyone had experience with this issue and know where to start? Also any experience with Masonry Cream products, and if this sounds like it might help?

Any help is greatly appreciate. Thanks