That updated file did improve things a bit ... I can now select and change themes. But changes are still not being saved, and when I select 'Publish' a new blank page appears with the URL: /webassist/plugins/styler/unload.php - Ignore this now as it doesn't happen any more (see below).
Update to this message (a day-or-so later):
I deleted all my powerstore files and the MySQL database. Re-created the database and re-loaded all the files to the server. Ran the install again.
When I went to either the powerstore home page or the admin page I got a page full of text. So I then uploaded the 'support fix for sparodic file permissions issues'.
Now everything works as it should .... except for one thing.
When I go to edit themes, the changes do not take effect. If, for example, I change the background to white, it just stays black. When I save the template nothing has changed, even though when I go back into the edit theme area it says it has a white background.