What kind of trades person do I need to look for?

If you have oil central heating with a small tank in the roof space, check that there is a cover on it. It is used to circulate hot water around your system and if there is no lid the water evaporates into the roof Space, down the inside of the roofing felt and usually onto the walls causing all sorts of damp problems. If it's not that have a look at your guttering see if they need cleaned or repaired. Failing that if the wet spots are near a window check the window seals and then the window cill make sure it's all good. If it's a new build and all these are good then there is a possibility the wall links are installed incorrect which means the outside water can travel across from the external wall to your inside wall (hopefully not this) older house, it might be a breakdown in the plaster but check the easy stuff first and rule them out. Hope this helps. One other thing if the spot is below any plumbing such as a radiator you need to check for leaks. John.

Insufficient information to even have as a guess, could be bad pointing,roof,dripping gutter,windows chimney to name but a few different, causes. Good luck Alex

Taking a pot shot on this one, is the wall in question exposed to the elements, ie? Not sheltered by another building or trees? Has the property recently or in the past had cavity wall insulation installed? Does that wall face South/South West, theres a good chance if youve had cavity wall insulation installed that it has been installed incorrectly, I can advise further with more information.. Regards Darren