It cannot be a result of this bad hotfix. This occurred two years ago on windows server 2008 so not relevant to what you are talking about. Also at the time, I was not updating the servers regularly - I was doing them manually and there had been no updates around that time.

I will try this again and see if I have an issue again.

I am glad to say that I am not as stupid as some people. Why the hell would anyone want to use public dns servers in dhcp - do these people not understand that active director and windows logon etc require internal DNS?

Anyway you have convinced me to have a look at using root hints again. I suspect that you do need to update windows update on servers for root hints to work all the time - its the only explanation I have for why several clients failed on the exact same time - no changes at any of the clients, no updates on the servers etc. So its the only thing I can conclude.

It will also help to reduce the load on the BT Router in the event that this was causing the problem, although not anymore because I put a new router in.