Quote From happyclappy:
Are you absolutely sure that this is the case, before you go down the route of accusing your supervisor in a tight-knit environment? If so, then you clearly need to act. My suggestion would be get the proof, then take it to your personal tutor or director of studies, and let them deal with it and find an alternative. I personally would not approach your scurrent, or intended new, supervisor. Good luck, what a tricky situation !
That's what I'd do, although I'm so grateful I didn't end up in such a situation. Good luck and keep us posted.