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Interesting! We have double glazing which probably doesn't help and so far have been drying clothes in the living room.
There was a damp stain on that wall when I stripped it back but it didn't go very high and didn't look very bad. The floor boards seemed fine too. Air bricks at the front are OK.Do you have one of those set back loos as well?
Well_is_it-
It looks like condensation - drying clothes will raise the humidity, double glazing will keep the moisture inside, the sofa will prevent air circulating, and the wall will be cold as it is expose to the outside and may have a heat bridge.
Moving the sofa may help.
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The trouble with external single skin brick walls is that they will get very cold in winter, so the water vapour from drying your clothes in the living room will condense in the cold spots. Behind a sofa is a common cold spot. In the past with open fires and leaky windows this wasn't so much of a problem because there was always good air flow, but nowadays with central heating, hermetically sealed double glazing and blocked up chimneys, it's a recipe for condensation.
Once you've ensured that there's no penetrating damp (as per rodabod above) I would suggest investing in a dehumidifier - you can get them for £100 or so - and leave it running in the living room whenever you're drying clothes there. Even without drying clothes you'll be amazed at the amount of water it collects. Alternatively open the windows and put up with losing a little heat. Also move the sofa away from the wall a bit.
The alternative to all this is to batten out the external walls and plasterboard it to create a pseudo-cavity, but this is annoying and will take away some room space, so see if you can solve it without doing that first!
I've got a 2 bed Warner so I think the layout is slightly different to yours, but the basic construction is the same. The loo is in the bathroom (which was originally a scullery, so the loo had a little partition wall to separate it).
edit: TW beat me to it :)
NotThamesWater
BobbyBriggs
I live in a ground floor Warner as well, and I have damp in a similar area. Rising damp is incredibly rare despite what surveyors tell you. It's most likely penetrating damp from leaky guttering and/or the wind blowing the rain into the porch area (this happens to me, very annoying!), potentially coupled with condensation due to poor air circulation too.
A few Qs: Do you know if your air bricks are blocked? What kind of windows do you have? How do you dry your clothes in the winter? Are the fireplaces blocked up?