I now see my post was not that clear. I was frustrated at the install and login because I've seen so many open-source projects lose ground due to the necessity for absolute clarity during the install process and initial stages (and I appreciate that is not Roundcubes fault, but what we've all come to expect).
So I appreciate the response. I was confused at first because SSL account details login was not working, but also the way Roundcube has been described and presented led me down the wrong path.
Roundcube is not an email client, not in the sense of Thunderbird and Outlook. I think most people use one of those as either a gateway to multiple accounts, or an email plus collaboration suite, or because their company uses Exchange. So Roundcube is not another option instead. So I had assumed initially one install would allow multiple accounts (as this is the setup most Thunderbird / Outlook users have), therefore it seem possible the login was not the email account.
Gmail is gaining round as an email client because of allowing access to multiple accounts. It was not generally called an email client before enabling multiple account features.
I think the way Roundcube being called an email client does suggest it could replace Thunderbird or Outlook, but it absolutely is not be a replacement, unless the user was a single account, not collaboration, non Exchange user (in my opinion, with Exchange their is no point in anything but Outlook, as it all depends on MS anyway).