Andrew Collins wrote in message ...
I suspect the problem is very minor but is being made a lot worse by
the fact that a membrane has been fitted between the brick and the
plaster up to a height of 1m. This means that there is a line of damp
at 1m accross my wall.

This could be coming down rather than up.
I and others have spouted quite a lot on the "damp" subject in the past as a
Google search will reveal. I suggest you look at that first.
It's a major problem in brick built Victorian houses and there is really no
quick fix. After years of pondering my own problems, I'm coming to the
conclusion that you can't stop it getting into the wall and the best you can
hope for is to stop it reaching your interior decorations.