Just for completeness...
I did log a bug with the team. This was the final outcome
The issue turned out to be a combination of a poor setup on the domain controller which was providing 127.0.0.1 as the DNS server address and a change in behaviour in FreeNAS.
Technically the loopback address was correct as the DC was also the DNS (presumably FreeNAS picked up the value during a handshake on startup) but was read literally by FreeNAS rather than converting it to the actual IP of the DC which it must have known to reach the DC in the first place. In 9.10-STABLE (my last working version prior to solving this problem) FreeNAS was getting its DNS information from the entered values on the network configuration page (and an examination of the network summary page showed the same values as those entered on the config page). After this version this seems to have changed in that an examination of the summary page was now showing 127.0.0.1 - a value which could only have come directly from the domain controller.
The solution was to modify the DNS value being provided by the DC and then restart FreeNAS
Hopefully this post will help others to avoid this quite squirrilly gotcha. :)