After spending a day reading the hundreds of posts on this topic I did find my answer, thanks to an earlier reply on 30 November 2012. That reply was for an earlier version so I thought I would update it for the current PRO version.

The issue was simply that although the thumbnails were being correctly generated in the cache folder, the browser (any flavour) was not permitted to view them. So as Fotis has posted elsewhere, it is often a matter of permissions - however it can be MORE than just folder and file permissions on the server. It can also be a form of configuration permission i.e. something in ".htaccess"

The line I needed was:
RewriteRule ^cache\/jw_sigpro/ - [L]

I added the following:
"cache/jw_sigpro"

in the Akeeba Admin Tools section:
"Server Protection / Exceptions / Allow direct access, except .php files, to these directories"