Yes I suspect I will end up asking a builder (the trouble is finding one who is willing to come and have a look at something that’s not necessarily a concrete project - excuse the pun).
Haven’t heard of anaglypta before, will look into this, thanks. Might wood chip be a fire hasard?
You won’t know what’s causing it before you or someone gets a hammer and bolster to it.
Looks like a wooden beam with part of the stairs using it for support. It could be all of the above mentioned… but could be that it was very dry and hot last Summer so the walls expanded a little and so did the beams but at different rates then contracted again at different rates. Or some sort of fungal rot. Humidity ingress. Etc.
Don’t go down the clip paper method as your just hiding the cracks for a while and they’ll be back and you haven’t solved the problem, in the case of any rot you’ve just given it more time to damage the rest of the house.
I didn’t mind Artex ceilings so much but covering walls with it was diabolical… the number of scratches it caused if your hand or arm scraped along the surface accidentally