The main reason for the glitch seemed to be some geometry which was not properly edged. I’d edited a probuilder pipe as part of the upper Labyrinth and I had missed filling a gap where I’d removed a plane. I know this because I rebuild the wall and the error hasn’t occurred again.
I also removed all but 2 sections of the night garden and added two pipes which reduced the polycount and processing load.
The redesign also resolved the issue of multiple sharp turns which could potentially make the player feel sick.
I also stopped the music from looping because as much as I like it, i’m pretty sure it could become irritating after a while!
Although I prefer the original spiral staircase it simply isn’t workable and therefore I will stay with this design until I can build something better. I could look at a larger, wider staircase with less steps and less garden flowers but I’m not sure it would have the same impact.