Black Mold, Damp and or Condensation Reappearing on Replastered Walls?

[Dampvictim]

One room with mold has previously been picked back to the wall and re plastered, but the black mold has returned, with the wall paper working its way off. First floor.
Another room (first floor), has a crumbling wall behind a radiator, again with the wall paper working off the walls.
The property was built in 1890, thick external walls, with what we have found to have nothing under the floor boards other than earth.
We have only recently spent a large amount of money re-rendering all the exterior walls and placing a new roof on top of the green papered room, but as far as I can see we have spots of what I call "damp" in a a number of rooms, but I'm no expert.
I've had a number of builders and professional damp companies and nobody tells me the same and more importantly how to remedy the problem, indefinitely!
Please help, Diolch.

[welsh brickie]

Damp spots can occur due to lack of ventilation in the room this sounds like the most probable cause, it may be necessary to install an airbrick, and have trickle vents installed in the windows to allow air to circulate, as heat rises from the radiators and hits cold air from the thick walls, condensation occurs causing mould to appear

[Dampvictim]

Thank you Welsh brickie, the walls are to thick for an air brick, but I shall certainly look into,the window vents.
What about the treatment of the walls, where the wall paper has gone black underneath. Also, what about the patches where the plaster has flaked off to the bare wall.
How should I treat to prevent it happening again?
Thank you,

[welsh brickie]

Use neat bleach to remove the mould and hire a dehumidifier to dry the room out

[Gary6537847]

If it wasn't rendered with a breathable material this could be the cause.

Sand/cement render doesn't allow your bricks to breath if they need to, which they will if built using soft brick and/or line render.

The above is only what I've found out online trough some research.