Further update:
Ran php on the index.php from the command line.
It output the following errors.
phpmyadmin: Failed to load /var/lib/phpmyadmin/blowfish_secret.inc.php Check
group www-data has read access and open_basedir restrictions.
phpmyadmin: Failed to load /var/lib/phpmyadmin/config.inc.php Check group
www-data has read access and open_basedir restrictions.
phpmyadmin: Failed to load /etc/phpmyadmin/config-db.php Check group
www-data has read access and open_basedir restrictions.
I checked the permissions of the files mentioned; they have read access by
group www-data.
I don't know what is meant by 'open_basedir restrictions'.
Captured the output HTML from running php and attempted to display it in the
browser.
It showed nothing (as expected) so edited bits out until it did.
The item causing the blank page was this:
[style]html{display: none;}[/style]
[ I do not know how to display quoted code in this forum, so have replaced
the angle-brackets round the HTML tags with square brackets, otherwise it
fails to display properly on the forum page. ]
Removing the 'style' tag allowed a reasonable looking page to display.
I'm not an expert on HTML, so not sure why that tag is there,
possibly it is meant to be overridden by some other included text that is
missing.
The page that showed had a couple of possibly significant lines:
Javascript must be enabled past this point
and
The $cfg['PmaAbsoluteUri'] directive MUST be set in your configuration
file!
Anyone able to throw any light here?
