Are PhD students considered employees under employment rights law. I would like to know what legal avenues (and any other avenues) are available for a PhD student who is bullied by their supervisor.
I would also welcome any response from any PhD students who are and have been bullied by their supervisor for advice on how to deal with such a situation that occurs in the absence of witnesses.

This is a hard question to answer, since if you think about it, there are an infinite number of equally valid hypotheses that can be generated in a given research situation.

Don't let it get to you too much, and most of all don't listen to everything that everyone says (including me). I have problems with my supervisor, but they are personality clashes and at the end of the day you must be bold enough to go and sort them out for yourself. Complaints are a last resort in an academic environment. I'm not saying you can't complain, but you have to exhaust all other possibilities first. Be diplomatic and tell your supervisor how you feel, then tell him/her why what he/she does makes you fell that way, and then tell him/her what you would like to change/happen. If that doesnt' work, then get a member of staff that you know and trust and tell them, then get them to have an informal meeting with all present and get that member of staff to mediate. If that doesnt work then you can speak informally to the head of department, speak to human resources, a university councillor, a student representative or even your doctor (I would reccommend a doctor first for advice). But most of all, your situation is unique to you, so everying I say may not be wise and/or useful.