Our first thought was that we hadn't let the plaster dry for long enough or that the new paint was absorbing moisture from the steam, but then we noti ced it was worse after it rained (and, boy, did it rain in late winter/spri ng).
We've had two roofers check our tiles and a builder look at the wall. The r oofers found nothing wrong and the builder suggested that the double-skin b ricks in our 1920s house had no cavity above ground-floor level because of a recess for pebble dashing.
I've just been out to look at what happens in heavy rain (getting soaked in the process) and noticed the lie of the tiles, with the lowest tile being at a less sharp angle to the others, leaving a line just above the brick pl inth. See picture at http://twitpic.com/e83zlm
There was a drip coming from this line but I wonder whether this is also tr ickling water into the wall. The main area of damp is about 2-3ft away from the plinth, although there is a small area of damp at the very top of the plinth on the inside.
My theory now is that the water is trickling in but because the plinth is d ouble-skin with cavity, the water is seeping into the wall nearby. I know t hat with flat roofs the damp can often be feet away from the leak.
Any views on this idea, please, and any ideas on how to remedy this?