sAx;
Thanks for the thoughtful and informative response. Rather than take the thread off-course, if I may, I would like to ask a couple of questions by pm. I'm trained as a captain and don't have a background in statistics, finance or engineering but would very much like to pursue one or two points raised which I think I need to understand. I had to look up "ambit" - wonderful word!
Kind thanks,
PJ2