Thanks fro your suggestion. It was actually set to NULL which was defaulting to PLAIN as the recommended setting for my server. I changed the setting to LOGIN and am receiving the same error, both on the login page and also in the error logs.
For completeness I tried the other auth types, however these trigger an unsupported authentication mechanism error in the logs.
Out of interest I entered an incorrect password and still receive the same onscreen and error log errors. Initially I had thought the problem was in creating a user account in Roundcube, but now I suspect there's a problem connecting to the IMAP server. Login did work during the installation proc4ss.
Any other suggestions?