Follow up to previous post - I repeated my previous efforts over the weekend, and got 2.3 working eventually.
Only difference was a completely clean install of 12.04 B1 from a CD rather than installing 11.10 and the do-release-upgrade -d.
Everything else was exactly the same, same network set up, same users, same passwords etc - got to assume that there was something broken in the 11.10-12.04 Upgrade path?
Not a comprehensive answer, and a lot of starting from scratch - I am really interested in Zentyal functionality and 2.2 would do the job, but there's a hardware compatability issue with my test rig and pre 11.10 version of server, so only option is 2.3 (except for 2.2 in a VM on a >11.10 Server which I did also get working, but not sure if the memory/processor is man enough for that job)