Damp wall - what's causing it?

paintpot

I just thought I would update everyone on this one as lots of people kindly took the time to offer advice.

In the end I had all of the plaster taken off the wall. I also had some bricks removed to ensure that there were no old lead pipes hidden in there that could have been leaking but there wasn't. It was then replastered with specialist plaster which is meant to prevent damp coming through, I can't remember it's exact name. At first, I wasn't sure that this had worked as it seemed that the damp patches "moved". However, I think might just have been the drying out process as it is now decorated and nothing has come through. It's been a few weeks now and we've had some heavy rain in that time. Fingers crossed it stays that way.