It's adjacent to a conservatory separated from the house by a bifold door. So I opened up a hole in the plaster board to check the cavity. Our 1930s house has foam cavity insulation (previous owner). And there's a cavity area with no foam roughly next to the damp area.
There's also a big draft blowing up from the 60cm air void under the suspended timber floor.
There is no sign of mould / dampness on the cavity side of the internal skin of bricks, yet the plaster is sopping wet in the room.
I suspect the cavity wall void is creating a cold spot and hence the condensation. It's got a lot worse over the last 4 years though. And we haven't had any major work done.
So I think the answer is to fill the void in the cavity insulation ..?
There's another damp area about 4 metres away on a dividing wall, but I guess same issue (though there's probably no cavity in a dividing wall I guess).
The picture shows the hole into the cavity where the insulation void is, the damp plaster and the dry looking cavity.
- Picture of cavity void