Thanks bellini13, I have read through the spec that it is PHP 7 compatible, but not found that PHP 7 is the minimum required, unfortunately the Documentation link from the install page (which says "PrestaShop compatibility with your system environment has been verified!" in a nice green box with a tick) .... goes to 1.6 requirements not 1.7 (not an issue, expected with a 0.0 release)
I will keep at it for a short while, PHP 7 is an option, but my customers run a variety of software on their hosts and I can't jump them to PHP 7 just yet. So that will be a complete stop for me showing 1.7 to clients. though I will still set it up for me, wait for the 0.1 version and try again...