Cheers TB - that's what I was after!

I also imagine that these things are a load of Betty Swallox.  But I'm a scientist, and I know that as soon as I catch myself saying "it can't possibly work because of the following fixed preconceptions...", I have definitely wandered off into woo-woo land. 

So I was hoping for some facts from experience.  Ideally an impartial trial done by somebody outside the fuel industry - and the magnet industry!  Or, like TB, someone who has actually tried it for a few months and can confirm it makes no difference.