This is where I'm at today.
PXL_20240421_135604524 by
The Pingus, on Flickr
Friday I was thinking about removing the quadrant, but had started to think that was a bad idea by the end of the day.
Today I decided to strip all the paint off the top of the stringer and quadrant to get a better look. The quadrant in the photo above still has a trail of paint on it in this photo til I get a scraper with the right profile.
Now I've tidied up, I've just been inspecting the gap with a torch and a straight edge. If I lie the straight edge down the wall I can see that the bottom of the plaster has been built out by quite a lot to meet the edge of the quadrant. That seems like a frikkin stupid idea.
Anyway, what I'm thinking now is I should remove that quadrant, make good the plaster so that it is flat to below the top of the stringer (this will be several mm behind the stringer) and when that's all dry, get a new, thicker bit of quadrant and glue it to the wall (so that it's not really in contact with the stringer.
Thoughts?