I have worked on many properties with damp problems and often the most effective solution is to dry line the walls, where battens are fixed to the existing damp wall and plasterboard is then fixed to the battens creating a cavity. This prevents the damp penetrating the new wall, which is then plastered and painted. The beauty of this, is that there is now space to put insulation in the new cavity which will increase the heating efficiency of the room. Thus turning a cold damp room into a lovely cosy living space. See the photos below of the process step by step. In this particular example, I have tanked the damp wall first with a Bituminous paint for added protection.