I’d posted a query few weeks back about Preview not reproducing the colours properly whereas in Photoshop the colours looked fine. (http://macmost.com/forum-poor-colour-reproduction-in-preview.html) I’ve stumbled upon what the problem was, so in case others are interested, here’s the issue.
I was creating my Photoshop files in CMYK colour mod, which when exported to JPEG was poorly reproducing itself in Preview.
In RGB mode, this problem doesn’t crop up. I have no idea why this should be, my light/colour theory is weak, but in case anyone has the same issues, just remember to create in RGB mode.