Just as an update on this, hopefully of use to someone.... there have been a couple of updates and I now have a better understanding of what is happening and to fix.
1. When installing pihole, choose YES to install admin interface and NO to install lighttpd
2. The snakeoil firmware upgrade process replaces the contents of /var/www, deleting all pihole web admin files (not data)
3. Workaround. Make a copy of /var/www/html somewhere else (home directory)
- after upgrade, copy ~/html back to /var/www
For pihole admin, use the full path
http://x.x.x.x/html/admin/index.php
For snakeoil admin use
http://x.x.x.x/