The title postgraduate researcher
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.