Simon,
I know it's old! It's just out IT dept not updating and i need a good reson to get them updated ( i personally am used to the Mac version)
Yes, it's when we save as jpg and use the slider it seems to have little effect, I vaguely remember a discussion about it years ago!!
There may indeed have been a bug in that version related to JPEG compression values. I'm sorry but I just can't remember that far back.
4.2.1 was the last version in the 4.2 series and there were no further point releases.
There are a large number of improvements, more camera support, features, etc. in the 4.5.x version and I think they would find it to be a worthy upgrade.
-Kirk