Thanks folks.
I sent a note to iOptron, and they have come back (reasonably quickly) and informed me that the plate solve may fail if the pole is more than six degrees away from the main field of view of the iPolar camera. That was a new one...
I didn't honestly think I was that far off target - the skies were clear and I know my way around the night sky - but anything is possible, so I plan to try again when the skies clear. Whenever that may be. I'll let you know how it goes. Humble thanks for the advice.
