As a follow-up to my own post, I found the likely configuration file that seems to set the default pool path at:-
I edited the "path" line in that file to reflect my preferred pool path (It was still pointing to the default path of "/var/lib/libvirt/images" despite the fact that the UCR variable "/uvmm/pool/default/path" was set to "/data/vmstore"). The edit survived a reboot & solved my issue.
So either my UCS config is slightly broken, or perhaps this could be a bug. I hesitate to suggest a bug as I'd be more than a little surprised if I were the 1st person to run across it. Maybe someone would like to compare notes on this and confirm/disprove? If I haven't seen a response in the next few days I'll run up a test install when I have more time to see if I can reproduce it.