Svejk, I'll keep my £100, but get in touch with James Randi and do the same thing and you can get £1 million. Good luck! Like everyone else, I'd expect you to fail, but if you succeed in controlled conditions then you'd deserve every penny for such a massive breakthrough.
Naomi, I half-wonder if the plastic might make some difference, too. But on the other hand the dowsers who were tested didn't seem to think it would matter. Also, some dowsers seem to think that they can divine with pretty much anything, be it metal or wood or glass, so I doubt that the material makes any difference whatever.
There's no mechanism for this to work, and there's no evidence that it does beyond personal accounts. Which are fine to set up a study -- but then when the study draws a blank and people ignore that, well, what can you do?
Never mind. Ultimately this is about personal experience versus scientific study, and pretty much everyone gives more weight to their own experiences. Not surprising, really, I'm sure there are times I do it myself -- but personal experience is not something you can base a credible opinion on, because it's so vulnerable to human flaws.