The issue bringing up the admin interface turned out to be a corrupted uci config file preventing the web server starting up:
/etc/config/uhttpdPossibly unrelated, but this might actually have been caused by problems introduced way back with the original 1.51 SP1 code in
/etc/itus/ituswebfilter.sh
You will find a
couple of instances of this comment missing a leading # character:
Add blacklist to dnsmasq
My admin interface came back again after restoring an old
/etc/config/uhttpd and running
sh /etc/itus/ituswebfilter.sh
The fix has been committed to
the GitHub repo, along with a bunch of others I've collated from @roadrunnere42, @user8446 and @wisiwyg along the way. Hope this helps anyone else with a similar issue.
OpenWrt SNAPSHOT, r10391-3d8d528939