How can I stop damp coming through my walls?
If you can get to the outside face - apply a water repellent product which will rainproof brick, render or masonry. These type of materials are based on special silicones called siloxanes and are like a Scotchgard or Gore-tex effect on the surface – waterproof but fully breathable.
A major by-product of treating walls like this is that as they dry out they become fantastic insulators and give a huge reduction in heat and energy loss through the masonry. This can be as high as 40-50% reduction, depending on your wall construction. You can reduce your carbon footprint easily with this approach.