Neil, that wasn't a light-hearted comment, it was a dickish comment. Saying it's lighthearted is similar to a school-yard bully saying he's just having fun. Certainly the forum should be fun for you, but you have to be polite and allow others to enjoy themselves as well. Furthermore, the ultimate point of this forum is to further Flat Earth Theory, not to amuse you.
So. Cavendish Experiment. I admit I'm not an expert on this one, I believe Tom Bishop would probably be the best candidate for answering your question, but I believe that his response is that Gravity is not the only universal force, even in RET. There's also the weak and strong nuclear forces and more importantly Electromagnetic force. Electromagnetism is not accounted for in Cavendish and indeed experimenters use metal balls in the experiment. These metal balls are denser and therefore more affected by the supposed gravity, but they are also more subject to static and other electromagentic phenomena.
Also, just saying, but you made this thread at noon on a Wednesday. Most FE'ers are going to be at work right now. I'm only able to answer your question because I'm home with the flu. Have a bit of patience.