Stairs stringer moving - how to fix?

[higherorder]

My stairs are developing quite a big gap between the skirting and the wall. When stepped on they make a clunk sound and the gap gets wider. Under the stairs are enclosed with plasterboard for a cupboard so hard to see what's going on. Any idea what a fix might entail?

 

[Wayners]

Check floor is good. I've fixed a few where the concrete floor was hacked at to lower the level so that the stairs lineup at the top landing.
This hacking at the concrete is a mess and bottom step gets worse with movement with nothing substantial under.
You can lift carpet to look at that.
If that's fine then remove second step or remove plasterboard under stairs to gain access and fix.
Here is one I worked on.
I managed to get my head and shoulders under to repair above and below this entry point before closing off.