Likely this was from a UK PD library in the late Eighties but I can't be definitive on that.
Program is Listed at end of video.
https://www.youtube.com/watch?v=Ef4S6Ng7HvI&t=2s
Links:
https://cpcrulez.fr/demostestSL_zx81_screens.htm
https://en.wikipedia.org/wiki/ZX81
Not emulated - original hardware and software.
Please note that the aspect ratio for this YouTube video is 16:9 but the CPC monitor
has an aspect ratio of 4:3 so you may wish to adjust your viewing device accordingly.
Someone clearly had a lot of time on their hands!