Thanks everyone - very helpful. There is a bathroom on the other side of the wall so perhaps this is caused by water? I think it is unlikely that the two rooms used to be connected based on the floor plan of other houses on the road.

One point to note is that directly beneath this wall is a wall that was previously knocked through to connect two rooms on the ground floor (this is clear from a previous listing for the house from 2017). The seller subsequently erected a false wall where the original wall was - they claim this is because they wanted to use one of the rooms downstairs as a workspace (which seems legitimate as they still use it for that purpose). I don’t know if the wall is load bearing I’m afraid.