I hate to be a wet blanket, but bear with me for a while.
..
In 1994 I replaced a coal boiler with a gas combi. In 1999
water was running down the living room wall. After eliminating
the obvious candidates, slates, flaunching, pointing, flashing
etc I was left with only one option - the flue (the products of
combustion are carbon dioxide and water). This proved to be the
case. It had been damaged on installation and, for five years had
been pumping water into a 75cm shale wall with soil/lime mortar.
The flue was replaced.
..
Six years on, the wall is still not completely dry despite my having,
this year, chopped off and repointed the bottom metre of external
rendering and run a 2kw fire and a dehumidifier in the sealed room for
8 - 10 hours a day.
..
Another side effect (this is speculation)is that the formerly sound
rendering has been loosened by winter frost action on the moisture in
the wall, so I may have to replace this as well.
..
The moral of this story is to assume nothing about the behaviour of
damp. All you can do is find its source, block it, do what you can to
dry it, then wait, wait, wait.
..
The mystery, to me, is why, in this age of technological wizardry, when
probes can be landed on other planets and the evolution of a life can
be tracked from ejaculation to delivery, there is no simple hand held
device which, either electronically or seismologically, can detect the
precise location of moisture in a wall. Moisture meters are a nonsense:
they tell you what you can see or, by running your hand over the wall,
feel, but tell you nothing (without drilling) about what is happening
within it.

Tony.