Sean has solved what was a very persistent damp (or so I thought) problem for me. He also did this for a very reasonable price, coming out on a very cold day.
When he came to view the job, he suggested that the issue might not simply be rain penetrating the new paintwork, to the interior (the wall is only single skin but it's a very thick wall, and given the level of water that was getting in, it simply didn't make sense for it to be coming directly through the wall). Sean suggested it might be gaps around the windows.
When Sean actually got up to have a look, his suspicisions were confirmed. The problem was being caused by rotten wood on the window sub frames (I have double glazing, but set into the original outer sub frames). He sealed and painted around the sub frames, and even took some photos for me.
To sum up, he could have suggested an expensive interior re-plaster job, but he didn't; I would recommend Sean without any hesitation.