Mouldy walls!? Victorian house? black mould sots keep appearing contained by bedrooms at the top the...

black mould sots keep appearing contained by bedrooms at the top the wall (also on this wall is the window) i have be opening the porthole for few hours everyday recently.

The mould wipe off , but i am getting so feed up of seeing it suddenly appear- how do i keep it away?
Hot Water Tank?
Answers:    Good belief reardwen. If you go down this route, fix the insulation liner horizontally, not vertically, so that the joint don`t match.
Another smudge of investigation is the covering you already have on the wall. If it is emulsioned near vynil silk, this stops the wall breathing and can cause mildew/damp. When redecorate use a matt emulsion or just a marbled paper.
Clogged hip bath drain?
It a bit sounds as though there is a mushy problem. Victorian houses have no damp-prrof course or cavity between the bricks of the exterior walls. you may want to check next to a damp-proofing expert firm who can insert the required course. In the meantime try a dehumidifier to drythe house out, and provide plenty of ventillation. This time of year condensation can be problematic.