Thanks Suavi,
the culprit was, well, my own stupidity.
I was sure all variables were the same, but when I examined the FITS headers, I realised that somehow I'd managed to take one exposure at 1 second and the other at 10 seconds, hence the difference.
Duh.
I can only assume that I either adjusted the exposure or entered it wrong. Not impossible as I was more interested in getting fields to solve for focal length calculations than comparative ADU.
Ahem, as you were.
Markus
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