> I was wondering have there been any sequel novels published for the UFO
> series (like the way Powys Media published the excellent Space-1999
> continuation novels which I highly recommend)?
I'm not aware of any sequel novels. Just the official two from the 1970s.
There has been a lot of UFO fan fiction written over the years, initially published in fanzines and then more recently published online. For example, "the SHADO library" as a lot of fan fiction:
https://www.shadolibrary.org/library/title.shtmlMarc