"Ed_Zep" wrote:
So can anyone tell me how to diagnose a damp interior wall please?

The only DIY method I know is to fix a small piece of glass onto the plaster
with a circle of putty or plasticine so that there is a gap between the
plaster and the glass. If there is moisture in the trapped air then it will
condense out on the inner surface of the glass. This test will confirm damp
but doesn't tell you its source. But as your walls have been re-plastered
very recently there is no point in doing this until the new plaster has had
chance to dry out. Didn't the installer tell you not to paint/redecorate for
at least 6 weeks?

Can condensation really be the cause of very damp bricks.

Seems unlikely. Condensation often causes damp plaster and mould growth.

What is the
standard solution for that?

Well, there's http://www.axp.mdx.ac.uk/~john49/dampfaq.htm
Scroll down to condensation.

And there's what the damp proofing companies will tell you, which may or may
not be correct.