modify wpmu registration email

Hi, I have been trying for days to modify the registration email that is sent out by WP when a new user has activated their account, to no avail.

I have tried hooking into wpmu_signup_user_notification_email and wpmu_signup_user_notification_subject but this does not work. I have successfully modified the wpmu activation email using add_filter('wpmu_signup_blog_notification_email', 'myprefix_change_activation_email_body', 10, 8); and add_filter('wpmu_signup_blog_notification_subject', 'myprefix_change_activation_email_subject', 10, 8); but the filters do not work for the above.

Can anyone please help?

Thanks in advance.
Graham J OConnor