Drupal Multisite

Trying hard to get simple multisite to work
Read ALL the manuals and doco, and read the forums.
I've done EXACTLY what they all say but still get the following error every time I try to log on to the sub-site ..
"The requested URL /sub was not found on this server."

My setup is ..
Two databases in MySQL .. a) griffo, and b) sub_griffo
password on both is 'grif', and have granted all rights on both to 'russell'

drupal is in /var/www/html/drupal
/sites directory has default, griffo, and sub_griffo folders.

settings.php in griffo has three pertinent lines ..
$db_url = 'mysql://russell:grif@localhost/griffo';
$db_prefix = '';
$base_url = 'http://griffo.com';

settings.php in griffo.sub .. the three lines are
$db_url = 'mysql://russell:grif@localhost/griffo';
$db_prefix = 'sub_';

in 'default', the lines are
$db_url = 'mysqli://russell:grif@localhost/griffo';
$db_prefix = '';
the database url line is left hatched out in this one

Apache is configured with
DocumentRoot "/var/www/html"
and no virtual servers configured.

I have set /etc/hosts to resolve griffo.com as 127.0.0.1

I've tried it withand without the 'default' folder,
as I understand that all I need is the main (griffo) and subsite (sub.griffo)
to be able to run two sites, griffo.com, and griffo.com/sub

but whatever I do, I just get the same error .. the url not found for the /sub

have I overlooked something crucial?
I feel I'm a click or two away from success

Appreciate any advice/assistance

Thanks in advance

Russell Griffiths