I think I've fixed it...
I've changed the local & root settings as suggested, but I was still getting the same sort of error messages on-screen when trying to navigate from admin/index.php using the nav tabs at the top of the screen.
So... I've been through content_insert.php, content_results.php, content_detail.php and in code view have changed all instances of "localhost" to "cms" and that seems to have sorted it - at least, the pages are displaying properly on-screen and I've added some test content and can view it, so my plan is to go through the other admin folder pages and make this a global change.
One thing I would like to clarify though, is this. I have a smallish (12 pages of static HTML) site which I need to convert so the client can update the content themselves (which is why I went for PowerCMS in the first place). I want to use the existing site content exactly as is, and want to use the existing CSS for the site rather than whatever comes bundled with PowerCMS. How exactly do I do that?