Just a bump to say this is an ongoing problem, however it has morphed into no user information being presented to squid1/dansguardian/squid2
I set up the same server at home in a virtual machine and could not get the authentication to break.
I then restored the config from the work server and have managed to get the problems occuring again, I'm quite happy with this progress at least.
Somehow the whole squid auth with kerberos is failing.
I'm trying to get the info from the logs, but so far no luck.