Subdomain installs with same users

Hi,

I have installed wp on a handful of subdomains, using the same database with different db prefixes. So far so good.

I then tried to set them to use the same userbase. This is where I have come unstuck.

I have added the lines:

define('CUSTOM_USER_TABLE', 'hu_users');
define('CUSTOM_USER_META_TABLE', 'hu_usermeta');

to my config file. My problem then is getting the admin from the ‘main’ shared userbase access to administer the other installs. Users seem to be logged in ok, but are sent from the login to the admin which fails, giving

You do not have sufficient permissions to access this page.

They are logged in, but wrongly directed, and in the case of admins, not to the correct role.

Any ideas?