My undergrad certificate is like that, could have a degree in anything!!

Hi I'd just talk to them. I was in a very similar position and just talked to my current masters supervisor and told her I'd been offer a phd project at another (better) uni that was much more what I want. She was a bit disappointed, but understood. I think just talk to them, the sooner you do it the more time they have to find someone else. As another lecturer told told me before i told my supervisor, they should understand and want the best for you, if not your definately doing the right thing by leaving.