Guys, that was it! Importing the file as a Library worked. I removed all previous related articles and libraries, before I did so. Now a lovely L-shaped hole in the floor. Bit odd that I can't get rid of a strip of carpet where the stairs touch the wall (despite the lower wall being raised higher than necessary and the stairs moved as close to the wall as permitted). I'm happy with the result though and look forward to getting this into my Home Assistant (automation) set up.

Thanks all!