Thanks for the super fast reply Jared.
I have disabled the “syncs add to model” (or similar) check box because I understnad the model can interfere with the centering in SGP. There was a massive thread on this which I’m still reading through 
I’ll double check the JNow and J2000 settings to ensure they are consistent.
Another thing to try is to clear the Gemini model just before running the centering (or check all values are zero’d). It’s possible that previous plate-solves have somehow added to a model. The consensus seems to be that having a model can cause problems, and there’s no need for it if using plate-solving.
It seems that this is not an uncommon problem, even with high-end mounts: Platesolving not working - Sequence Generator - Main Sequence Software
I’ll try some of these suggestions, and post my updates.
Thanks and best regards,
John Wheeler