Did the plasterer do any repairs in that corner, like hack back to the brick and render, bond it out? That's the coldest point in the room, what can happen during the drying process is that all of the moisture from the plaster drying out will condense in that area, maybe it was a problem area before it was skimmed and the new plaster is showing it more now. Was there bubbling or salting in the area before he skimmed the wall. Things like this are a pain in the arse, especially on older properties.