I removed a piece of plasterboard on a side to have a look below the staircase
I don’t know if the wedges between risers and treads are original or if someone tried to fix it in the past
What I noticed is that there is some vertical movement of a tread where it joint the following riser (circled in the 3rd picture)
I have only got access to these first three steps, everything else would involve removing other pieces of ceiling and reskimming
Is there any chance of driving more screws from the outside?