Thanks Nick, this sounds very much like what I’m looking for. As for using the CLI, that’s what I generally do anyway.
This morning I kludged up a Perl script to modify the CRVAL[12] cards in the FITS headers in attempt to persuade SWarp to stack the images with an offset. Not very successful though. Ether I screwed up the code or the total exposure just wasn’t long enough because I can’t see anything circular on the stacked image — just lots of trailed stars.