bp wrote:
I live in an end of terrace house. Upstairs in the middle bedroom I
can see what looks like mould in the corner. It is black stuff that
does not wipe off, towards the floor, and skirting. It goes up about
2ft up the wall.

I am about to decorate the room so my question is. What are possible
causes of this. How can I remove it and what shoud I be doing to
prevent it re-occuring.

It's probably from where some furniture stood close to the wall,
restricting ventilation.

Go over it with some ordinary bleach ("thin" bleach, not lavatory
cleaner), dry it off, and decorate.

I have looked at the corner externally (not on a ladder just from
ground level) and cannot see anything that looks odd (i.e. loose bricks
etc). However there is SKY dish fixed to the wall further up and the
wire is tacked down the wall near that area. Could this be causing it.

Anything that lets water drip or run down the wall won't help, but
that does not sound significant.