Very weird, Robin. I used to do astrometric alignments in Maxim and never had any problems like that. These days I use PI which works brilliantly, of course
If you can put them up on your web site temporarily I'd be happy to have a play with the master calibration files and a light frame to see if I can figure out what is going on. I seem to spend a lot of time poking around in FITS files these days.
Cheers,
Rick.