User error on my part. I'm so used to hitting Ctrl-0 to resize the image, I hadn't twigged that after making it 960px wide, Ctrl-0 is making it bigger than 100% on my 24" monitor.
Just tried both ways and at 100% they are identical. I'll get used to hitting Ctrl-1 (100% zoom) instead.
(also, whilst I'm here, these two pages aren't showing for me - tried two computers and same on both, so they may have gone walkabout https://www.damiensymonds.net/art_tscs2.html and https://www.damiensymonds.net/art_tscs000.html )
Thanks as always,
Andy