Clive Long,UK wrote:

To summarise what I have found.

The wall is 7.60m long, 2.35m high. It is a "shared" (with adjacent
flat) internal wall, probably load bearing as concrete lintels cross

Do you know what is the other side of the wall? (for example a bathroom,
a radiator, aquarium ;-) ?

The skirting measures between 17 to 24. The skirting is more damp the
closer the reading is made to the junction with the floor.

The wall immediately above the skirting measures between 20 to 50.
The highest wall reading occurs directly above the highest skirting
reading.
Question: Could the high conductance be due to damp wall-paper rather
than the plaster?

Well it can be - those meters can be rather notorious for measuring all
sorts of unexpected things if you don't pay attention to how you are
using it (i.e. conductive wallpaper, surface moisture, cinder block
(i.e. high carbon) wall construction). They were originally designed for
making measurements in timber - for which they are quite accurate -
hence your comment about a high moisture content on the skirting would
seem to indicate more than just surface moisture.

Initial thoughts

1. The radiator is causing the problem. But how? And what to do? I
can't move the rad.

The radiator itself may not be a problem - you would probably see water
dripping / spraying out if it was leaking - possibly even rusty water
stains, but where do the pipes go?

For example, a set of pipes buried in the concrete floor can be a soggy
mess waiting to happen if the pipes were not adequately protected before
being concreted over.

2. Damp is coming from the floor. How do I investigate that further?

Short of drilling a hole and analysing the water content of the dust
removed by the drill - tricky.

Any ideas on what to investigate next to try to isolate the problem
before I call someone in who may attempt the wrong treatment because
they haven't understood the cause ?

Look for the obvious causes first. So called "rising damp" is very rare
in reality - almost unheard of in modern building materials. Penetrating
dampness from the other side of the wall is far more likely. So check
that with the neighbour (also finding out if they have the same problem
(or a less or more severe version of it) would help. See if you can
trace where the radiator pipes go as well.

--
Cheers,

John.

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