you should really fit a air cover on the chimney breast, scrape the back
stuff off the walls and check its not damp inside the wall,
www.toolstation.com sell a Damp checker for around £9 the check upto around
third of a inch into your wall for damp, if it is you need to dry that out
(not a easy task)

If not wash the walls down with anti fungal paint, (B&Q) leave for a few
days and cover as you desire...

Now you need to allow air into the room, the easiest but not cheapest option
to change the pain of glass that has air inlet fitted. if you have just
blocked the fireplace up, make a hole and fit a air hole cover.

hope this helps...
"Zinzan" wrote in message
...
I have two rooms that seem to suffer badly from condensation. On cold
mornings the Windows are covered in condensation and over a period of time
the painted walls start to turn black. Is there anything I can do to cure
this? Is there are special paint that I can paint onto the outside or
inside
walls to stop the walls from turning this colour. One room used to have a
fireplace but I have now blocked this up. The other room a bed is up
against
the wall that shows signs of condensation. Any ideas would be appreciated.

Thanks