The title postgraduate researcher I have seen the title postgraduate researcher in a couple of peoples Linkedin profiles for roles in the UK. The people listed only had masters degree before showing. What is this position formally, is it just someone who has not completed a PhD? Or are these official positions in subjects like astronomy?
AFAIK, postgraduate research would, equally, be known as both Ph.D. and post-doctoral programs. Graduate studies do cover M.Sc. programs, there. Therefore, whatever after that would be named as postgraduate. So, one might assert that a postgraduate researcher could be either a Ph.D. candidate or a post-doctoral fellow.

Of course, theses definitions are highly flexible and even interchangeable among different universities, even in UK.