Thanks for your reply. Pics of the water marks internally attached. They exactly match some of the cracks, and when it rains they drip a bit. It's not general condensation. They are all on the first floor - so far can't see any on the ground floor.
I'll see if I can find something long enough...
My house (built 1950ish) is non-cavity wall and is mostly rendered externally. Render was redone about 4 years ago. This winter, lots of horizontal cracks have started appearing along one wall, about 2 feet apart, at ground and first floor level (this wall is exposed and west facing, and takes...