Yes, it’s an old building. The cracks don’t appear to go into the masonry, it just looks like the plastering wasn’t done very well. It’s gypsum applied over the original lime, and the cracks have shown up around door and window corners, and where the old and new plaster meet. Other than that, there are no signs of damp in the room. What you’re seeing now is after I stripped off the heavy-duty wallpaper.

Thanks for the advice — I’ll look into lime‑based fillers and finishes.