Hi Benson,
Thanks for your reply. Yeah my contours were imported as part of a DWG, spent along time carefully putting the contours back together. Since the problem of the site model opening up damaged a couple of attempts, I have not had any problems since. Im sure that i did everything the same and it managed to hold its feet.
It just feels to me like one of those mysteries that me as an amateur, will stumble upon!
Thanks for your time